Implement support for file: includes in OWNERS files.
This CL implements support for file: include lines in OWNERS files,
both as top-level directives and as per-file directives. The
paths can be either relative or absolute.
Examples of lines in OWNERS files:
file:test/OWNERS (relative, top-level)
file://content/OWNERS (absolute, top-level)
per-file mock_impl.h=file:test/OWNERS (relative, per-file)
per-file mock_impl.h=file://content/OWNERS (absolute, per-file)
A whole series of tests to cover this feature have been added
to owners_unittest.py as well.