package-inferred-systems and primary-system-name

[Eric Timmons]

If a have a package-inferred-systems "a" and "a/b/c", the following
code used to return "a":

(primary-system-name (find-system "a/b/c"))

But after commit 069cd2a6 it returns nil.

Happy to patch it, but I wanted to check how to do it before starting.
The root cause right now is that system-source-file returns nil for
the inferred systems. The easiest fix would be to have it return the
asd file for the primary system. That's probably not *technically*
correct since the system isn't actually defined in that file, but it's
probably correct in the principle of least surprise sense.

Thoughts?

-Eric