ww: select build action from package declaration

test/wcc/data/r79_xmod_valglobal_def/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 5
+package main;
 package aa;
 export def K: i32 = 5;
 export fn getk() i32 = { return K; };