fix PantsTargetReferenceSet so that it has correct behavior when tabbing from path:

Before, it would throw an offset error. Now it correctly lists the completions. It also correctly completes the target instead of appending the selected completion

eg
   foo:b<caret>, then select 'bar' would result in foo:bbar.

Reviewed at https://rbcommons.com/s/twitter/r/1333/
refactored PartialTargetAddress
Dependency quick fix for Scala

Testing Done:
Tests are passing

Reviewed at https://rbcommons.com/s/twitter/r/1400/