Remove support for __file__ in BUILDs.

NB: this is a BUILD API break.

Fixup BuildConfigurationTest to match removal of __file__ and fixup code formatting.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/42913383

Bugs closed: 835

Reviewed at https://rbcommons.com/s/twitter/r/1419/