Part 7 - implement Cross.Factory macro (TODO split shims out)

- fix scanning of Cross modules in Discover macro
- improvement: check factory value matches crossValueN

Author: bishabosha

example/depth/cross/6-axes-extension/build.mill

@@ -56,6 +56,9 @@ trait FooModule3 extends FooModule2 with Cross.Module3[String, Int, Boolean] {
 
 > mill show foo3[b,2,false].param3
 error: ...object foo3 extends Cross[FooModule3](("a", 1), ("b", 2))
-error: ...                                      ^
-error: ...value _3 is not a member of (String, Int)
-*/
+error: ...                                      ^^^^^^^^
+error: ...expected at least 3 elements, got 2...
+error: ...object foo3 extends Cross[FooModule3](("a", 1), ("b", 2))
+error: ...                                                ^^^^^^^^
+error: ...expected at least 3 elements, got 2...
+*/