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/wcc/cgendecl.ww

@@ -612,7 +612,7 @@ fn cgfn(c: *cgen, fn_: *syntax.node) void = {
 	cgout_flush();
 };
 
-export fn cgfile(c: *cgen, file: *syntax.node) void = {
+fn cgfile(c: *cgen, file: *syntax.node) void = {
 	if (file == nil) { return; };
 	c.strlits = nil;
 	c.strlitseq = 0;