[Extensions] Add entry_points information to ExtensionModuleEntry

Now that we have which entry points are supported by each extension, it makes sense
to keep it together to the other meta information that we keep about the extensions
in ExtensionModuleEntry.
[Extensions] Support multiple entry points in our trampoline

The process of installing or removing (in the moment the extension is
loaded) trampolines is the same no matter it is for a implicit entry
point (the extension name) or for "extra" entry points. So, we make
some helpers that make both cases the same.