Summary:
A common pattern when programming with signatures is to combine multiple
signatures together (signature linking).  We achieve this by making it
not-an-error to have multiple, distinct interface files for the same module
name, as long as they have the same backing implementation.  When a user
imports a module name, they get ALL matching signatures dumped into their
scope.

On the way, I refactored the module finder code, which now distinguishes
between exact finds (when you had a 'Module') and regular finds (when
you had a 'ModuleName').  I also refactored the package finder code to
use a Monoid instance on LookupResult to collect together various results.

ToDo: At the moment, if a signature is declared in the local package,
it completely overrides any remote signatures.  Eventually, we'll want
to also pull in the remote signatures (or even override the local signature,
if the full implementation is available.)  There are bunch of ToDos in the
code f...