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.

lib/math/random/random_test.ww

@@ -1,6 +1,6 @@
 package random_test;
 
-import random;
+import math.random;
 import test;
 
 @test fn seq() void = {