"compact" serializer is not fully compatible with "current" resolver

Here is an example:

• go to wiki:space.page and add a velocity script
• serialize "wiki:space.space2.page" using "compact" serializer
  -> you end up with "space2.page"
• if you resolve it using context serializer you ends up with "wiki:space2.page"

The issue is that "compact" serializer is based on reference element while it's expected to be based on element type IMO (it just used to be the same thing before nested spaces). At least for retro compatiblity we should make sure "compact" serializer compare the whole "space.space2" against "space", i.e. be based on entity type.