Windows: further improve module maps

Further enhance the modulemaps.  This is needed to support the use of
the ucrt for the builtins as used in the ClangImporter unit tests.

Author: compnerd

stdlib/public/Platform/ucrt.modulemap

@@ -99,9 +99,14 @@ module ucrt [system] {
     }
   }
 
-  module io {
-    header "corecrt_io.h"
+  module corecrt {
+    header "corecrt.h"
     export *
+
+    module io {
+      header "corecrt_io.h"
+      export *
+    }
   }
 }
 

stdlib/public/Platform/visualc.modulemap

@@ -21,6 +21,11 @@ module visualc [system] {
     export *
   }
 
+  module vcruntime {
+    header "vcruntime.h"
+    export *
+  }
+
   module stdint {
     header "stdint.h"
     export *