Fix use-ing of modules that are already transitively needed

TODO

@@ -99,8 +99,5 @@ Other stuff to do after:
   - $_ handling: declare in *every* sub, empty/defouter according to returnable
   - cnperl6 prototyping...
   - CLR: KISS and then play with async I/O, possibly including soric
-  - Fix up name handling
-  - package to class steal
-  - jnthnian packages
   - pm's = hack
   - fix { my $x } crash

src/Metamodel.pm6

@@ -782,7 +782,7 @@ class Unit {
     }
 
     method need_unit($u2name) {
-        return $.tdeps{$u2name} if $.tdeps{$u2name};
+        return %*units{$u2name} if $.tdeps{$u2name};
         my $u2 = %*units{$u2name} //= $*module_loader.($u2name);
         $.tdeps{$u2name} = [ $u2.filename, $u2.modtime ];
         my @new = $u2name;