Switch to using buildifier's -path option

[jparise]

Buildifier offers a -path command line option:

Buildifier's reformatting depends in part on the path to the file
relative to the workspace directory. Normally buildifier deduces
that path from the file names given, but the path can be given
explicitly with the -path argument. This is especially useful when
reformatting standard input, or in scripts that reformat a temporary
copy of a file.

This lets us drop our -type-guessing logic.

For the moment, we rely on the buffer filename being "relative to the
workspace directory", which it generally is, but in a future change, we
should introduce logic that will locate the WORKSPACE file and adjusts
the filename relative to that directory. This could be complicated based
on the working directory in which buildifier is executed, so a little
more research is necessary to implement that logic correctly.

[hsanson]

Thanks, this simplifies the fixer a lot.