ww: select build action from package declaration

test/wcc/data/r989_ampfncollide_loud/case.ww

@@ -1,4 +1,5 @@
 //ww:run-exit 42
+package main;
 // Current-module &fn lookup must beat a later same-leaf declaration.
 package beta;
 export type hfn = fn(x: i32) i32;