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/dyn.ww

@@ -4,12 +4,11 @@
 // exports and which DT_NEEDED entry to record. We do not pull bytes
 // from the .so; the dynamic loader maps it at runtime.
 
-package w6l;
+package main;
 
 import os;
 import rt;
 import strings;
-import sym;
 
 def ET_DYN_SO:     u16 = 3u16;
 def EM_X86_64_SO:  u16 = 62u16;