PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

• from foo import bar adds //foo as a dep instead of //foo/bar.
• from foo.bar import baz adds //foo/bar as a dep instead of //foo/bar:baz (when //foo/bar:baz is a separate py_library that contains foo/bar/baz.py as a source).
• from google.cloud import aiplatform does not add "@gazelle_python_test_google_cloud_aiplatform//:pkg", as a dep (fails to resolve from imports for pip packages).

Issue Number: #709

What is the new behavior?

When gazelle encounters from foo.bar import baz, try to resolve, in order from most specific to least specific:

foo.bar.baz
foo.bar
foo

This way, if foo.bar.baz is in the manifest file or in the index of existing rules (e.g. from a rule that contains the source file foo/bar/baz, it can be found). If foo.bar.baz can't be resolved, try foo.bar (maybe baz is just a function or variable in foo/bar.py).

Does this PR introduce a breaking change?

• Yes
• No - all existing tests continue to pass.

Other information

May want to consider antagonistic cases - are there cases where imports are truly ambiguous?