ww: select build action from package declaration

test/wcc/data/r921_amp_xmod_def/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 18
+package main;
 package myf;
 export type pt = struct { x: i32, y: i32 };
 export def P: pt = pt{x=7, y=11};