Make 'changed' faster by not instantiating already injected targets

Testing Done:
Travis passed. https://travis-ci.org/pantsbuild/pants/builds/85231805

Bugs closed: 2368

Reviewed at https://rbcommons.com/s/twitter/r/2967/

src/python/pants/build_graph/source_mapper.py

@@ -60,7 +60,7 @@ def _find_targets_for_source(self, source, build_files):
       address_map = self._address_mapper._address_map_from_spec_path(build_file.spec_path)
       for address, addressable in address_map.values():
         self._build_graph.inject_address_closure(address)
-        target = self._build_graph._target_addressable_to_target(address, addressable)
+        target = self._build_graph.get_target(address)
         sources = target.payload.get_field('sources')
         if sources and not isinstance(sources, DeferredSourcesField) and sources.matches(source):
           yield address