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/parse.ww

@@ -10,12 +10,10 @@
 //   instr := \tMNEM\t[OP1[, OP2]]
 //   OP    := $NUM | REG | NUM(REG) | (REG) | name(SB) | label
 
-package w6a;
+package main;
 
 import os;
 import strings;
-import lex;
-import opcodes;
 
 fn streqlit(p: *u8, n: u64, lit: str) bool = {
 	if (n != lit.len: u64) { return false; };