ww: select build action from package declaration

test/wcc/data/r929_match_6arm_shadowed/case.ww

@@ -1,4 +1,5 @@
 //ww:run
+package main;
 // migrated from test/wcc/929_match_4arm_cross_module_run.c: 6-arm — every arm beyond the caller's variant count was broken, not just arm 2/3.
 package a;
 export type more = void;