WIP: Gazelle filegroup generation and buildifier_test

[tanyabouman]

This PR adds

• an internal Gazelle extension which generates filegroups
• gazelle rules to use the extension
• the newly generated filegroups
• a buildifier_test rule to replace buildifier with check.

There are two type of filegroups. The all_files groups make all files in a directory available for the future haskell_bazel_test. These groups are not included in this PR. The starlark_files groups make BUILD, WORKSPACE and other Starlark files available for buildifier_test.

TODO: Gazelle doesn't work properly in nested workspaces.

For example, in examples/BUILD.bazel:

filegroup(
    name = "starlark_files",
    testonly = True,
    srcs = [
        "BUILD.bazel",
        "WORKSPACE",
        "//examples/bazel-bin:starlark_files",
        "//examples/bazel-testlogs:starlark_files",
        "//examples/cat_hs:starlark_files",
        "//examples/primitive:starlark_files",
        "//examples/rts:starlark_files",
        "//examples/transformers:starlark_files",
        "//examples/vector:starlark_files",
    ],

Two problems here:

• the srcs paths are relative to the top-level workspace, and not the nested one
• Gazelle is going into bazel-* directories, which it doesn't do at the top level

Things to try:

• reproduce a smaller example, and file an upstream feature request/bug report
• run Gazelle from within the nested workspaces (rules_haskell/examples$ bazel run //:gazelle_starlark_files), and don't let the top level run (rules_haskell$ bazel run //:gazelle_starlark_files) go into the nested workspace at all

TODO: Check that Gazelle is including the correct files and directories.

Some of these might need to be specified manually. For example, *.md should probably be excluded, even from all_files, and that doesn't appear to be happening yet.

TODO: something in the CI to run Gazelle to make sure that the filegroups are generated

TODO: try buildifer_test in the nested workspaces: examples, examples/arm, tutorial, tests/c2hs/repo