Summary:
move p4fastimport under hgext3rd as that's where it belongs.
Also has the benefit that we package it up.
Test Plan: rt test-p4*
Reviewers: #mercurial, quark
Reviewed By: quark
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5056664
Signature: t1:5056664:1494631758:4892f47922d8fbbd2c1f7793f3f29ee73fa8fa42
