ww: select build action from package declaration

test/wcc/data/r989_trycallcollide_control/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 42
+package main;
 package beta;
 export type berr = !void;
 export fn op() (i32 | berr) = { return 5; };