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/obj.ww

@@ -11,10 +11,9 @@
 //
 // Symtab indices: 0 = STN_UNDEF, 1.. = our syms. Only GLOBAL symbols.
 
-package w6a;
+package main;
 
 import os;
-import opcodes;
 
 // Local wrappers around os.writeall's tagged return — collapse the
 // (i64 | oserror) back to a boolean / int sentinel for the