Fix order-dependent force handling: if a version is forced in one place, it is forced everywhere.  Two JarLibraries might compare equal even though their force setting is different; we want force to be dominant.

Reviewed at https://rbcommons.com/s/twitter/r/1085/
Sync of https://github.com/pantsbuild/pants/commit/d90bdc0d4046a7d1b3d0c7b243ebc31de19bd9fb