When we auto-load a module because the user typed a qualified name at
the prompt, we better not auto-load a home interface (because we won't
have the code to go with it).

So, introduce a new constructor in the WhereFrom datatype, namely
ImportByCmdLine for these auto-imports, and make findAndReadIface fail
if it tries to load a home interface by this route.

ToDo: GHCi should *never* demand-load a home interface under any
circumstances, but we don't have an ASSERT for this yet.