Support flat package names

[daniel-sanche]

From issue report:

The Python generator throws a jinja2.exceptions.UndefinedError: No first item exception when a proto package name is "flat" (e.g., package backstory;) rather than hierarchical.
Resolution: Explicitly define the hierarchical namespace and package name in the opt_args of the py_gapic_library target.

[parthea]

We can add this feature if there is interest for APIs that follow AIP-191. For package backstory, I don't think it does.

https://google.aip.dev/191#single-package

Google APIs must reside in a directory that matches the protocol buffer package directive. For example, the package above dictates that the directory be google/cloud/translation/v3.

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.