Buildifier should respect .bazelignore

When I run `buildifier -r`, I think it should skip the same directories that Bazel itself skips.

Start by tracing the recursive directory traversal used by `buildifier -r` and how `.bazelignore` is represented or parsed in the repository. Compare the desired behavior with Bazel's skipped directories; the work is done when ignored directories are excluded from recursive formatting and the behavior is covered by the repository's tests.

Written by the indexing model from the issue text.