Make sure ModIface values are still forced even if not written

When we are not writing a ModIface to disk then the result can retain a
lot of stuff. For example, in the case I was debugging the DocDeclsMap
field was holding onto the entire HomePackageTable due to a single
unforced thunk. Therefore, now if we're not going to write the interface
then we still force deeply it in order to remove these thunks.

The fields in the data structure are not made strict because when we
read the field from the interface we don't want to load it immediately
as there are parts of an interface which are unused a lot of the time.

Also added a note to explain why not all the fields in a ModIface field
are strict.

The result of this is being able to load Agda in ghci and not leaking
information across subsequent reloads.

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