format.py: Use bazel build, support excludes

[npaun]

Adds a few features to format.py that improve compatibility with the internal repo:

• Adds excludes to the config, listing directories that should not be formatted
• Uses bazel build instead of bazel fetch, so that bazel creates some symlinks the internal repo expects to exist.
• Fixes a mistake in the filegroup config for formatters distributed as a http_archive