A graph_info backend.
Review Request #3256 - Created Dec. 15, 2015 and submitted
Contains all the tasks that perform computation
on the build graph, and that used to be in backend/core.
While in that code, rename and tag an integration test
that was masquerading as a unittest and taking about 25%
of the total unit test exec time.
With this change, the elimination of backend/core is almost
CI passes: https://travis-ci.org/pantsbuild/pants/builds/96881334
Address code review comments.