generate: python adds libraries for sources rules_dx binaries, tests or exports_files already own (part of #9237) #9333

Problem

On main (9a01083), bazel run //dx:generate_check proposes:

• tools/docs/rule_names/BUILD.bazel: a new python_library(name = "rule_names", srcs = ["rule_names.py"]), although rule_names.py is already the main/srcs of the hand-written python_binary rule_names_bin and python_test rule_names_test.
• python/rules/BUILD.bazel: a new python_library(name = "grpc_protoc") for grpc_protoc.py, which the package only exports with exports_files.
• python/tests/BUILD.bazel and python/tests/sdist_native/msvc/BUILD.bazel: testonly = True on existing python_test rules. Tests are already testonly, so this adds nothing.

Where

• gazelle/python/lang.go:465-485 foreignKinds/foreignSources: only rules_py/rules_python kinds count as owners.
• gazelle/python/lang.go:341-376: makes a library plan for every unowned source.
• gazelle/python/lang.go:385-387: FixtureTestonly sets testonly on every generated rule, test kinds included.

Fix

1. Treat a source as owned when it is in srcs or main of an existing rules_dx python_binary/python_test that the generator did not produce for that file, or when it is listed in exports_files. Do not plan a library for an owned source.
2. Skip testonly for testKind.
   Add one test per fix in gazelle/python/lang_test.go.

Done when

bazel run //dx:generate_check shows no change to these four BUILD files.