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.
Use the isScoped() bit to distinguish scoped imports from submodule
imports (both of which are split by null bytes in the string) so that we
don't try to lookup a submodule name as if it were a decl (leading to
assertion failures when it wasn't found).
This fixes interface generation of swift modules that import clang
submodules.
rdar://problem/24534122
