feat(gazelle) Remove entry point file requirements when generating rules (#2998)

Remove entry point file requirements when generating rules. Enable
python rule generation as long as there are .py source files under the
directory so all new packages will have python rules generated in the
package.

The extension used to require entrypoints for generation but:
- entry point for tests (i.e., `__test__.py` ) is no longer required
after https://github.com/bazel-contrib/rules_python/pull/999 and
https://github.com/bazel-contrib/rules_python/pull/2044
- entry point for binaries (i.e., `__main__.py` ) is no longer required
after https://github.com/bazel-contrib/rules_python/pull/1584

The entry point for libraries (`__init__.py` ) shouldn't be required
either, especially for Python 3.3 and after when namespace packages are
supported.

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Co-authored-by: yushan <yushan@uber.com>
Co-authored-by: Douglas Thor <dougthor42@users.noreply.github.com>