Summary:
Enhance check-ext script to be more strict:
- Only one foreign extension is allowed: `remotenames`
- Require explicit path for in-repo extensions to avoid wrong extensions
being tested
This would make the test more predicatable since system extensions
will be less likely to be imported. Explicit path is better than
setting `PYTHONPATH` since `hgext/name.py` could override
`hgext3rd/name.py` regardless of `PYTHONPATH`.
Test Plan: arc unit
Reviewers: phillco, durham, ikostia, #mercurial, stash
Reviewed By: stash
Subscribers: medson, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5271430
Signature: t1:5271430:1497861776:7dd35ec7c522cd9b26aa0871cb4306b4f1b8993a