Merge exported modules with the same public name in generated interface

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.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -764,7 +764,8 @@ static StringRef getModuleName(const ValueDecl *VD,
   // overlay's declaring module as the owning module.
   if (ModuleDecl *Declaring = MD->getDeclaringModuleIfCrossImportOverlay())
     MD = Declaring;
-  return MD->getNameStr();
+
+  return MD->getPublicModuleName(/*onlyIfImported=*/false).str();
 }
 
 struct DeclInfo {