RunCMake/CXXModules: fix partition importing syntax

Apparently GCC accepts the ill-formed prior syntax.

Tests/RunCMake/CXXModules/examples/internal-partitions/importable.cxx

@@ -1,5 +1,5 @@
 export module importable;
-import importable : internal_partition;
+import : internal_partition;
 
 #include "internal-partitions_export.h"
 

Tests/RunCMake/CXXModules/examples/partitions/importable.cxx

@@ -1,5 +1,5 @@
 export module importable;
-export import importable : partition;
+export import : partition;
 
 #include "partitions_export.h"