selfhost: migrate tool sources to directory packages

Build w6a and w6l from package-main directories and expose the wcc backend through a narrow package API so w6c and wwdump no longer import implementation files. Retarget the remaining load-bearing fixtures and example sources to directory packages; retain the one intentional flat compiler collision as an explicitly composed raw unit.

lib/math/checked/checked_test.ww

@@ -1,8 +1,8 @@
 package checked_test;
 
-// Directory import pulls both checked.ww and saturating.ww (the two
-// files of this module); a bare `import checked` would resolve only to
-// the checked.ww file. Referenced as `checked.*` (last path component).
+// The canonical directory import pulls both checked.ww and saturating.ww;
+// imported paths never select one source file. Referenced as `checked.*`
+// (last path component).
 import math.checked;
 import types;