rename dev_backend_loader to extension_loader

This is a subset of https://rbcommons.com/s/twitter/r/1429 but poor rename detection was making that diff much harder to read. Splitting this into its own change should make the added functionality and tests easier to identify in that change.

Testing Done:
./pants goal test tests/python/pants_test::
https://travis-ci.org/pantsbuild/pants/builds/43414631

Bugs closed: 849

Reviewed at https://rbcommons.com/s/twitter/r/1444/