Bazel: introduce buildifier formatting

This introduces tooling and enforcement for formatting bazel files.

The tooling is provided as a bazel run target from
[keith/buildifier-prebuilt](https://github.com/keith/buildifier-prebuilt).

This is used in a [`pre-commit`](https://pre-commit.com/) hook for those
having that installed. In turn this is used in a CI check. Relying on a
`pre-commit` action gives us easy checking that buildifying did not
change anything in the files and printing the diff, without having to
hand-roll the check ourselves.

This enforcement will make usage of gazelle easier, as gazelle itself
might reformat files, even outside of `go`. Having them properly
formatted will allow gazelle to leave them unchanged, without needing
to configure awkward exclude directives.

.pre-commit-config.yaml

@@ -20,13 +20,15 @@ repos:
       - id: autopep8
         files: ^misc/codegen/.*\.py
 
-  - repo: https://github.com/warchant/pre-commit-buildifier
-    rev: 0.0.2
-    hooks:
-      - id: buildifier
-
   - repo: local
     hooks:
+      - id: buildifier
+        name: Format bazel files
+        files: \.(bazel|bzl)
+        language: system
+        entry: bazel run //:buildifier
+        pass_filenames: false
+
       - id: codeql-format
         name: Fix QL file formatting
         files: \.qll?$