Move static library validation helper to `ar_files`

[fmeum]

trafficstars

It doesn't need to be staged for every compile or link action. The action that uses it stages all_files.

[fmeum]

A local run of buildifier -lint=warn -r . doesn't show any findings, but the CI run fails (without output). Not sure what to do about that.

[comius]

This breaks cc_builtins tests. https://storage.googleapis.com/bazel-untrusted-buildkite-artifacts/01931ae7-07f8-4c6c-b4b2-c951a9ac7410/src/main/starlark/tests/builtins_bzl/cc_builtin_tests/test_attempts/attempt_1.log

[fmeum]

I'll take a look at some point, but it's not a high priority improvement.