use registered ModuleLoader instead of hardcoding P6::ML

When parsing we invoked P6::ML before, and at runtime the registered one.
Now it is consistant and lets us register a custom ML.

src/Perl6/World.nqp

@@ -331,7 +331,7 @@ class Perl6::World is HLL::World {
     method load_module($/, $module_name, %opts, $cur_GLOBALish) {
         # Immediate loading.
         my $line   := HLL::Compiler.lineof($/.orig, $/.from, :cache(1));
-        my $module := Perl6::ModuleLoader.load_module($module_name, %opts,
+        my $module := nqp::gethllsym('perl6', 'ModuleLoader').load_module($module_name, %opts,
             $cur_GLOBALish, :$line);
 
         # During deserialization, ensure that we get this module loaded.