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.

include/swift/IDE/ModuleInterfacePrinting.h

@@ -38,11 +38,14 @@ namespace ide {
 /// Flags used when traversing a module for printing.
 enum class ModuleTraversal : unsigned {
   /// Visit modules even if their contents wouldn't be visible to name lookup.
-  VisitHidden     = 0x01,
+  VisitHidden = 0x01,
   /// Visit submodules.
   VisitSubmodules = 0x02,
   /// Skip the declarations in a Swift overlay module.
-  SkipOverlay     = 0x04,
+  SkipOverlay = 0x04,
+  /// Visit exported modules where their public module name matches the current
+  /// module.
+  VisitMatchingExported = 0x08,
 };
 
 /// Options used to describe the traversal of a module for printing.