fix(gazelle): embed default stdlib list for non-Bazel Go builds

[udaya2899]

gazelle/python/std_modules.go previously used //go:embed stdlib_list.txt, where stdlib_list.txt only existed as a Bazel copy_file output from @python_stdlib_list.

Building, listing, or testing github.com/bazel-contrib/rules_python/gazelle/python directly with standard Go tooling (go build, go test, go list -deps, or go install for standalone Gazelle binaries in pre-commit hooks) failed with pattern stdlib_list.txt: no matching files found, and running bazel run //:gazelle inside gazelle/ emitted a missing-file warning on std_modules.go.

Check in the default standard library module list at gazelle/python/stdlib_list/default.txt (verified via diff_test against @python_stdlib_list//:stdlib_list/lists/3.14.txt), change the Bazel copy_file output to stdlib_list/selected.txt, and embed stdlib_list/*.txt via embed.FS.

At initialization, loadStdModules prefers stdlib_list/selected.txt when present (preserving Bazel's python_version select() behavior) and falls back to stdlib_list/default.txt for non-Bazel Go builds.

Fixes #3821

[udaya2899]

827 lines of this huge looking PR is just the contents of the txt file. For prior art, https://github.com/EngFlow/gazelle_cc/blob/main/language/cc/bzldep-index.json checks-in a huge index json file

[aignas]

github.com/bazel-contrib/rules_python/gazelle/python directly with standard Go tooling

What APIs are you using? What are you building? I would have thought that there would be very little need to do this. :)

[udaya2899]

What APIs are you using? What are you building?

We want to run gazelle pre-push as a hook. We want to make sure people don't push code that doesn't adhere with gazelle. Currently the only way to do it is with bazel run //:gazelle for us, but gazelle being built with Go and having Go based language extensions, we can simply configure a go-based build which is way faster without the bazel setup costs. Fast enough to run pre-push atleast.