generate: python renames an existing python_library to the file-derived name (part of #9237) #9297

Problem

When a python_library already owns a source but has a name other than the one generate derives from the file, generate deletes it and adds a new target with the derived name. Labels that point at the old name break.

Found in #9279 (review on #9237): python/rules/BUILD.bazel has

python_library(
    name = "bench_plugin",
    srcs = ["dx_bench_plugin.py"],
    imports = ["."],
)

Generate replaces it with python_library(name = "dx_bench_plugin", ...), but python/rules/defs.bzl:140 adds Label("//python/rules:bench_plugin") to every python_benchmark, so all benchmarks (for example //python/tests/fixtures/hello:parse_bench) fail analysis.

Reproduction

On main (570927c):

tools/dx generate python/rules

proposes 4 edits to python/rules/BUILD.bazel; --apply removes bench_plugin and adds dx_bench_plugin.

Minimal case: a package with foo.py and python_library(name = "bar", srcs = ["foo.py"]). Generate renames it to foo.

Fix

Match existing rules by the sources they own before naming, as other extensions do for existing targets, so a library that already claims foo.py keeps its name.

Done when

Generate on main leaves bench_plugin in place.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Xut2hzpb4qWNZKpDCrwUz7