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.

selfhost/cmd/w6l/sym.ww

@@ -3,7 +3,7 @@
 // Singly-linked list, usually a few hundred entries; hashing isn't
 // worth it yet.
 
-package w6l;
+package main;
 
 import strings;