Distributed project.ignore

Today, you can tell buck2 to ignore files using `ignore =` in `[project]` in .buckconfig. Maybe we could support something like a `.buckignore` file (semantically equivalent to `.gitignore`/`.hgignore`)? If we could specify a spec for this filename in buckconfig and adopt .git/hgignore syntax, we could even reuse existing `.git/hgignore` files.

This differs from [`.bazelignore`](https://bazel.build/run/bazelrc#bazelignore) which is still only one file at the root (afaict).