Summary:
This commit moves most of the stuff in hgext3rd and related tests to
hg-crew/hgext and hg-crew/test respectively.
The things that are not moved are the ones which require some more complex
imports.
Depends on D6675309
Test Plan: - tests are failing at this commit, fixes are in the following commits
Reviewers: #sourcecontrol
Differential Revision: https://phabricator.intern.facebook.com/D6675329
