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/main.ww

@@ -7,11 +7,6 @@ package main;
 import os;
 import rt;
 import strings;
-import opcodes;
-import lex;
-import parse;
-import asm;
-import obj;
 
 fn cstreq(a: *u8, lit: str) bool = {
 	let n: u64 = lit.len: u64;