Add buildifier as a BUILD file formatter

[thejcannon]

This is the first plugin I actually question if it belongs here in the Pants repo, and not in another package. It's a bit of an abuse to run Bazel's Starlark formatter on Pants BUILD files, but:

1. I've basically never seen a syntax error in the wild, since BUILD files are relatively declarative. Only one I've seen is Starlark doesn't allow implicit string concat. I kinda like that 😛
2. Since we're looking towards Add new fmt plugins for build file formatting #16560 this is actually really useful for incremental adoption, where users who are using Pants for fomat+lint+check and Bazel for build+test and using two distinct files. We could fmt/lint their Bazel for them and show them how cool our caching is 😄 Until then, it's useful if you're doing the "two systems one file trick".

Additionally, this PR does not add any rules for running buildifier's lint capabilities as I don't think Pants would trigger those and not be a false positive. Once we implement #16560 it might be worthwhile.

[ci skip-rust]
[ci skip-build-wheels]

[stuhood]

This is the first plugin I actually question if it belongs here in the Pants repo, and not in another package.

I'm fine with having it in repo. Until the plugin API is stable, a burden is imposed on external plugins as APIs change. Post API stabilization, we can figure out whether to externalize plugins en-masse.

[thejcannon]

This is the first plugin I actually question if it belongs here in the Pants repo, and not in another package.

I'm fine with having it in repo. Until the plugin API is stable, a burden is imposed on external plugins as APIs change. Post API stabilization, we can figure out whether to externalize plugins en-masse.

I've thought about this more. I think the best middle-groud to all this is in-repo but out of pants-packaged. So dogfooding the monorepo-but-multi-dist support.

[Eric-Arellano]

I think the best middle-groud to all this is in-repo but out of pants-packaged. So dogfooding the monorepo-but-multi-dist support.

That was the v1 approach to all plugins. I don't see a ton of benefit to this: the versioning of the plugin is closely coupled to pantsbuild.pants itself. As long as a backend doesn't add new requirements, then there seems to be little cost to adding to pantsbuild.pants (trivially bigger package size).