partial fix for #1119

partial fix for #1119
Unless we're in one-shot mode, emit an error if we attempt to
demand-load interfaces for home modules.  This can only happen in one
way (that I'm aware of): typing a qualified name at the GHCi prompt
that refers to a module that isn't loaded.  Previously you got a
cryptic message about not finding an interface file, now you get:

Prelude> Foo.a

<interactive>:1:0:
    attempting to use module `Foo' (Foo.hs) which is not loaded

Of course you can still refer to package modules like this without
loading them explicitly, only home modules are affected, and the
behaviour is exactly the same as if you try to ':browse Foo' and
Foo isn't loaded.