Don't display extensions twice in the Core Extensions section after manually installing a JAR file

Steps to reproduce:

1. Install the "Headers Authenticator for XWiki" extension from EM
2. Uninstall it
3. Stop XWiki
4. Copy the JAR file from here to your "webapps/xwiki/WEB-INF/lib" folder
5. Start XWiki
6. Go to Administration: Core Extensions and search for "headers"

The "Headers Authenticator for XWiki v2.0.1" is listed twice.