Summary:
Port of D6798134 to fbsource. It eliminates module-import failures as well as errors like this:
```
mercurial.error.ForeignImportError: hgext.extlib.treedirstate: /home/phillco/.local/lib/python2.7/site-packages/hgext/extlib/treedirstate.so lives outside /..../hg
```
....that block other tests, like test-help.t
(Note: this ignores all push blocking failures!)
Reviewed By: quark-zju
Differential Revision: D6799259
fbshipit-source-id: b77d1b565dbf52165e0847002be498648658e064
Summary: Moves the extension itself.
Test Plan:
make local && ./run-tests.py
A future patch fixes the tests once
fastmanifest is also moved.
Reviewers: phillco, #mercurial
Reviewed By: phillco
Differential Revision: https://phabricator.intern.facebook.com/D6684580
Signature: 6684580:1515521896:68a4d648d9f5433d6eeda8be01740fbe12abf44e