In IvyImports, make a single call to ivy to map all jar files.

If you have many targets with jars to map in, this is a significant performance increase.

Testing Done:
Added a new unit test.
Ran this in our internal repo where we have a lot of uses of java_protobuf_library() using imports=.

CI running at:  https://travis-ci.org/pantsbuild/pants/builds/41531811

Bugs closed: 286, 809

Reviewed at https://rbcommons.com/s/twitter/r/1364/