If a module has the same `public-module-name` as the module being
generated and its import is exported, merge it into the same generated
interface.
Fix various always-imported modules from being printed while here and
update all the tests that checked for them.
Resolves rdar://137887712.
We used to compute the mangled name in other cases, but document structure is
a syntactic request and can't guarantee that the class/protocol we're getting
the mangled name of is valid in any way so it often breaks assumptions in the
mangler and causes it to crash. It's not clear if the runtime_name is actually
being used anymore, so this change restricts reporting it to just the cases
where we don't need to mangle.
rdar://problem/40956377
