Rework getReexportedModules to optionally find all imported modules.

...instead of just those that are re-exported. This will be used for
autolinking (and probably few other places).

As part of this, we get two name changes:
  (1) Module::getReexportedModules -> getImportedModules
  (2) TranslationUnit::getImportedModules -> getImports

The latter doesn't just get modules-plus-access-paths; it also includes
whether or not the import is re-exported. Mainly, though, it just didn't
seem like a good idea to overload this name when the two functions aren't
really related.

No tests yet, will come with autolinking.

Swift SVN r7487

include/swift/Serialization/SerializedModuleLoader.h

@@ -83,9 +83,10 @@ public:
   virtual OperatorDecl *lookupOperator(Module *module, Identifier name,
                                        DeclKind fixity) override;
 
-  virtual void getReexportedModules(
+  virtual void getImportedModules(
     const Module *module,
-    SmallVectorImpl<Module::ImportedModule> &exports) override;
+    SmallVectorImpl<Module::ImportedModule> &imports,
+    bool includePrivate) override;
 
   virtual void lookupVisibleDecls(const Module *module,
                                   Module::AccessPathTy accessPath,