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/wwi.ww

@@ -903,7 +903,7 @@ fn wwiemitfactimports(fd: i32, file: *syntax.node, owner: str) void = {
 	};
 };
 
-export fn wwiemit(c: *checker, file: *syntax.node, path: str) i32 = {
+fn wwiemit(c: *checker, file: *syntax.node, path: str) i32 = {
 	// §5: check_exported_type FIRST, before any byte — a producer
 	// without it can emit a dangling `.wwi`.
 	let bad: i32 = 0;