Summary:
Moves the remotefilelog extension into hgext/ and it's tests into
tests/.
I did not fix up all the check-module errors, since it's a ton of work for
very little impact at this point.
Test Plan: make local && ./run-tests.py
Reviewers: #mercurial
Differential Revision: https://phabricator.intern.facebook.com/D6680030
