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.

test/wcc/data/paramshadowmod/crossmod/crossmod.ww

@@ -0,0 +1,11 @@
+// crossmod — directory package whose probe() takes a param named like the
+// module `shadowmod` that paramshadowmod (NOT crossmod) imports. The
+// shadow rule is filtered by the binding's own module (src_imports
+// cur_mod filter): crossmod carries no `import shadowmod`, so this param
+// must NOT trip even though a sibling package imports that leaf.
+
+package crossmod;
+
+export fn probe(shadowmod: str) i32 = {
+	return shadowmod.len;
+};