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/w6a/opcodes.ww

@@ -1,6 +1,6 @@
 // Mirrors cmd/w6a/a.h and cmd/w6c/6.out.h.
 
-package w6a;
+package main;
 
 // Registers + operand kinds must stay numerically aligned with the
 // 6.out.h C enum so that ww-cgen output (which reads them via