ww: select build action from package declaration

test/wcc/data/r79_xmod_valglobal_f64/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 2
+package main;
 package aa;
 export let f: f64 = 2.5;
 export fn getf() f64 = { return f; };