Recompilation checking: ModSummary cache leads to incorrect results in multiple home unit scenarios

The ModSumary cache assumes that each file maps to one Module. With MHUs, this isn't true, each file can be used multiple times, as long as it is only used within one unit once.

One may question whether it is a good idea to allow the same file to be compiled multiple times within one session. However, this is how people have set-up their testsuites in cabal for generations (so internal code can be used for testing), therefore it's better to allow it whilst it doesn't cause any issues.

Therefore we just need to update the key of the cache to also contain a UnitId and we're all good.

This can lead to quite confusing errors on reload in GHCi (when the cache is used).