We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".
Fixes a longstanding issue where submodules with the same name in
different top-level modules weren't being sorted deterministically.
This doesn't come up very much in practice, and it would have been
hard to notice anything wrong, but it's good to be right.
