Fix broken external BuildLabel filename parsing (#257)
Fix broken external BuildLabel filename parsing (#257)

External BuildLabels were keeping their // halfway through their path, leading to projects xcode would refuse to load.

As an example: @google_toolbox_for_mac//Foundation:BUILD was going to external//Foundation/BUILD, not external/Foundation/BUILD.
This lead the pbjproj to assign the file the path //Foundation/BUILD, which Xcode refuses to load, along with groups named '/' and the omission of other associated source files.
bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi