Store interfaces in ModIfaceCache more directly
9.4 backport of just this part of the patch which changes the API.
I realised hydration was completely irrelavant for this cache because
the ModDetails are pruned from the result. So now it simplifies things a
lot to just store the ModIface and Linkable, which we can put into the
cache straight away rather than wait for the final version of a
HomeModInfo to appear.