Summary:
This is here to support other arcanist/phabrication integration
modules.
Note: this adds a new package. I'm not sure what the packaging ramifications
are exactly, but surely there are some as other extensions start to depend on
this.
Test Plan:
integration test is provided:
```
$ ../../hg-crew/tests/run-tests.py -j8
....................................................
# Ran 52 tests, 0 skipped, 0 warned, 0 failed.
```
Reviewers: #sourcecontrol, ttung
Subscribers: mjpieters
Differential Revision: https://phabricator.fb.com/D3215615
