Allow extensions to indicate features for dependencies

The issue was discovered in https://github.com/xwiki/xwiki-platform/commit/d8d6bc8d67db86d80282dbe7e73fb86590196187#diff-0.

It's easy to indicate EM what are the previous ids of some extension. But it's not really possible when we don't control the extension (like some extension coming from Maven Central).

In the case of the WAR, we workaround this by providing xed files along with each JAR file. But for installed dependencies we don't really have any dynamic solution, and ideally it should be something indicated by one of the backward dependencies (i.e. what is aware of those old ids).