rename export -> copy

Summary:
Export kind of makes sense (it's an SVN command that does a similar
thing) but it's not worth the confusion with our "export" field in modules,
which is a more important concept. "copy" seems like a reasonable name that's
less confusing.

Test Plan: Update integration test.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D53