Refactor into Python package format (#1165)

* Refactor into a python module format

-Updates import statements to be absolute vs implicitly relative
-Uses import syntax that minimizes the need to update references in code
and/or reformat affected lines (e.g. `import gapi.directory` becomes `from gam.gapi import directory as
gapi_directory`)
-Adds a `__main__.py` such that the module can be executed on its own
using standard `python3 -m gam` syntax
-Replaces __main__ import hack with module import
-Updates the GAM path to be the module's parent dir

* Add gam.py to /src for backwards compatibility

A stub that calls gam.__main__.main() to be used by users who are not
with the syntax of calling a module implementation. It should also
provide immediate backwards-compatibility with existing scripts with
references to this file.

* Move build tools back to the main dir and out of the package

* Fix pylint errors

* Update build spec to use new package format

Incorporates @jay0lee's patch from
https://github.com/jay0lee/GAM/pull/1165#issuecomment-618430828

src/gam/fileutils.py

@@ -0,0 +1,179 @@
+"""Common file operations."""
+
+import io
+import os
+import sys
+
+from gam import controlflow
+from gam import display
+from gam.var import GM_Globals
+from gam.var import GM_SYS_ENCODING
+from gam.var import UTF8_SIG
+
+
+def _open_file(filename, mode, encoding=None, newline=None):
+  """Opens a file with no error handling."""
+  # Determine which encoding to use
+  if 'b' in mode:
+    encoding = None
+  elif not encoding:
+    encoding = GM_Globals[GM_SYS_ENCODING]
+  elif 'r' in mode and encoding.lower().replace('-', '') == 'utf8':
+    encoding = UTF8_SIG
+
+  return open(
+      os.path.expanduser(filename), mode, newline=newline, encoding=encoding)
+
+
+def open_file(filename,
+              mode='r',
+              encoding=None,
+              newline=None,
+              strip_utf_bom=False):
+  """Opens a file.
+
+  Args:
+    filename: String, the name of the file to open, or '-' to use stdin/stdout,
+      to read/write, depending on the mode param, respectively.
+    mode: String, the common file mode to open the file with. Default is read.
+    encoding: String, the name of the encoding used to decode or encode the
+      file. This should only be used in text mode.
+    newline: See param description in
+        https://docs.python.org/3.7/library/functions.html#open
+    strip_utf_bom: Boolean, True if the file being opened should seek past the
+      UTF Byte Order Mark before being returned.
+        See more: https://en.wikipedia.org/wiki/UTF-8#Byte_order_mark
+
+  Returns:
+    The opened file.
+  """
+  try:
+    if filename == '-':
+      # Read from stdin, rather than a file
+      if 'r' in mode:
+        return io.StringIO(str(sys.stdin.read()))
+      return sys.stdout
+
+    # Open a file on disk
+    f = _open_file(filename, mode, newline=newline, encoding=encoding)
+    if strip_utf_bom:
+      utf_bom = u'\ufeff'
+      has_bom = False
+
+      if 'b' in mode:
+        has_bom = f.read(3).decode('UTF-8') == utf_bom
+      elif f.encoding and not f.encoding.lower().startswith('utf'):
+        # Convert UTF BOM into ISO-8859-1 via Bytes
+        utf8_bom_bytes = utf_bom.encode('UTF-8')
+        iso_8859_1_bom = utf8_bom_bytes.decode('iso-8859-1').encode(
+            'iso-8859-1')
+        has_bom = f.read(3).encode('iso-8859-1', 'replace') == iso_8859_1_bom
+      else:
+        has_bom = f.read(1) == utf_bom
+
+      if not has_bom:
+        f.seek(0)
+
+    return f
+
+  except IOError as e:
+    controlflow.system_error_exit(6, e)
+
+
+def close_file(f, force_flush=False):
+  """Closes a file.
+
+  Args:
+    f: The file to close
+    force_flush: Flush file to disk emptying Python and OS caches. See:
+       https://stackoverflow.com/a/13762137/1503886
+
+  Returns:
+    Boolean, True if the file was successfully closed. False if an error
+        was encountered while closing.
+  """
+  if force_flush:
+    f.flush()
+    os.fsync(f.fileno())
+  try:
+    f.close()
+    return True
+  except IOError as e:
+    display.print_error(e)
+    return False
+
+
+def read_file(filename,
+              mode='r',
+              encoding=None,
+              newline=None,
+              continue_on_error=False,
+              display_errors=True):
+  """Reads a file from disk.
+
+  Args:
+    filename: String, the path of the file to open from disk, or "-" to read
+      from stdin.
+    mode: String, the mode in which to open the file.
+    encoding: String, the name of the encoding used to decode or encode the
+      file. This should only be used in text mode.
+    newline: See param description in
+        https://docs.python.org/3.7/library/functions.html#open
+    continue_on_error: Boolean, If True, suppresses any IO errors and returns to
+      the caller without any externalities.
+    display_errors: Boolean, If True, prints error messages when errors are
+      encountered and continue_on_error is True.
+
+  Returns:
+    The contents of the file, or stdin if filename == "-". Returns None if
+    an error is encountered and continue_on_errors is True.
+  """
+  try:
+    if filename == '-':
+      # Read from stdin, rather than a file.
+      return str(sys.stdin.read())
+
+    with _open_file(filename, mode, newline=newline, encoding=encoding) as f:
+      return f.read()
+
+  except IOError as e:
+    if continue_on_error:
+      if display_errors:
+        display.print_warning(e)
+      return None
+    controlflow.system_error_exit(6, e)
+  except (LookupError, UnicodeDecodeError, UnicodeError) as e:
+    controlflow.system_error_exit(2, str(e))
+
+
+def write_file(filename,
+               data,
+               mode='w',
+               continue_on_error=False,
+               display_errors=True):
+  """Writes data to a file.
+
+  Args:
+    filename: String, the path of the file to write to disk.
+    data: Serializable data to write to the file.
+    mode: String, the mode in which to open the file and write to it.
+    continue_on_error: Boolean, If True, suppresses any IO errors and returns to
+      the caller without any externalities.
+    display_errors: Boolean, If True, prints error messages when errors are
+      encountered and continue_on_error is True.
+
+  Returns:
+    Boolean, True if the write operation succeeded, or False if not.
+  """
+  try:
+    with _open_file(filename, mode) as f:
+      f.write(data)
+    return True
+
+  except IOError as e:
+    if continue_on_error:
+      if display_errors:
+        display.print_error(e)
+      return False
+    else:
+      controlflow.system_error_exit(6, e)