EM should not allow installing package exposing an installed feature

Here is simple use case based on legacy-oldcore which expose oldcore.
Deploy your wiki with a non-legacy oldcore and try to install an extension that depends on legacy-oldcore.
Currently, it will install legacy-oldcore as a dependency of this extension.

We should not allow installing the extension in first place. We cannot support installing different "flavor" of a package that expose the same feature. In our use case, we cannot install an extension requiring a legacy core, when the wiki is deployed with a non-legacy oldcore.