Miscellaneous fixes to protobuf codegen.

Sorted protobuf paths and resources, and handled collisions deterministically. This process generates
copious warning/error log output where appropriate, as protobuf collisions are something the user should
definitely be aware of.

The imports property of java_protobuf_library was changed slightly to not cache its list of jars if it doesn't
manage to resolve one of its targets. This solves a problem where it incorrectly saves an empty list if it's
poked before the build graph loads.

Modified the log outputs of ivy imports to be more accurate/helpful.

Reduced occurance of extraneous/incorrect 'unconsumed gen targets' warning (it still happens, not as much).

Testing Done:
Tests under pants_test/tasks:: behave the identically to their counterparts on master.

Tested successfully on many square internal projects.

Bugs closed: 376

Reviewed at https://rbcommons.com/s/twitter/r/720/