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/dynout.ww

@@ -22,12 +22,11 @@
 //   [gotplt_off]   .got.plt   (writable; mapped by PT_LOAD #2)
 //   [dynamic_off]  .dynamic   (writable; covered by PT_DYNAMIC)
 
-package w6l;
+package main;
 
 import os;
 import rt;
 import strings;
-import sym;
 
 def ET_EXEC_D:     u16 = 2u16;
 def EM_X86_64_D:   u16 = 62u16;