docs: mention mvn2pants

while I was in there: setup_repo had a paragraph about top-dir BUILD files for config. LOL, nowadays, that config has moved into plugins. So pull that out, and re-jigger examples of BUILD.foo files.

Reviewed at https://rbcommons.com/s/twitter/r/1301/
Remove a superfluous test.

It only tests that Goal.parse_args works, but that function is
about to be killed itself.

Reviewed at https://rbcommons.com/s/twitter/r/1304/