Simplify BUILD.bazel parsing in `.generator/parse_googleapis_content.py`

[parthea]

See feedback from @ohmayr in https://github.com/googleapis/google-cloud-python/pull/14386#discussion_r2335159106 referring to the code block below:

https://github.com/googleapis/google-cloud-python/blob/0308e9d7461cd1591da807d02a068a7bb08e5778/.generator/parse_googleapis_content.py#L18-L73

The name of the bazel build rules are hardcoded in parse_googleapis_content.py. An alternate, more robust approach would be to read the bazel rules from BUILD.bazel files rather than relying on the hardcoded rules in parse_googleapis_content.py.

This tracking bug will be used to follow up making this improvement