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/obj.ww

@@ -1,11 +1,10 @@
 // Port of cmd/w6l/obj.c.
 
-package w6l;
+package main;
 
 import os;
 import rt;
 import strings;
-import sym;
 
 def ET_REL:        i32 = 1;
 def EM_X86_64:     i32 = 62;