Rewrite Buildifier support (#1129)
There have been many problems with Buildifier on Bazel CI, mostly stemming from the fact that Buildifier has evolved a lot, but our CI support hasn't kept up.
Ideally there would have been many smaller commits over time, but well...
This commit implements the following changes:
1. The code now uses a single Buildifier invocation for linting and format checks.
2. We now call Buildifier with --format=json, thus removing the need for error-prone parsing of unstructured text.
3. The old code checked stderr for Buildifier warnings, but at some point Buildifier started printing them to stdout only.
4. Finally we no longer determine the list of source files that should be passed to Buildifier, but simply use Buildifier's recursive mode ("-r .").
Progress towards #1080
