Keep same-prefix packages in external dependency output - #290

Closes #146.

pydeps --externals foo drops foobar because the external dependency scan treats every name starting with foo as internal. The same check also lets sources from foobar contribute their imports as if they belonged to foo.

Match the target package itself or its dotted submodules in both checks. This keeps foobar external while leaving foo.internal internal, and keeps the result limited to direct imports from the target.

Tests use actual temporary packages and module discovery:

• A foo package importing foobar.b now lists foobar.
• When foobar.b imports baz, output still contains only foobar; an internal foo.internal import stays excluded.

Validation on Python 3.12.14/Linux:

• New owning tests fail on the unmodified base: 2 failed, 2 passed.
• Patched owning suite: 4 passed.
• Full suite: 87 passed, 2 skipped.
• Changed production file passes Flake8 with the CI line-length setting.
• Sphinx HTML and nitpicky man builds pass with warnings treated as errors.