Adds support for extensions that changed name in package.json.

If an extension adds a package.json with a name that is different
from its pre-package.json GitHub-derived name, the user would
previously end up with two extensions. This change will delete
the old extension.
Adds test file needed for legacy package test.
Merge pull request #4149 from adobe/dangoor/update-with-new-name

Adds support for extensions that changed name in package.json.