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/asm.ww

@@ -1,11 +1,10 @@
 // Port of cmd/w6a/asm.c. Encoding subset matches what w6c emits — see
 // cmd/w6a/asm.c for the authoritative list.
 
-package w6a;
+package main;
 
 import os;
 import rt;
-import opcodes;
 import strings;
 
 export fn emitbyte(a: *asm_, b: u8) void = {