ww: select build action from package declaration

test/wcc/data/r84_qualstruct_fields/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 37
+package main;
 package pkg;
 export type point = struct { x: i64, y: i64 };
 export type rect = struct { lo: point, hi: point };