ww: select build action from package declaration

test/tool/rejects_test.ww

@@ -19,7 +19,7 @@ package rejects_test;
 // paramshadow (#19/#16) — value names and module names are disjoint:
 // a param/let/mlet/for-range/match-case binding named `shadowmod` in
 // a file importing module shadowmod is rejected at the decl site; the
-// renamed binding builds+runs 42 (no over-trigger); a param named
+// renamed binding in a real `package main` builds+runs 42 (no over-trigger); a param named
 // like a module a SIBLING module imports does not trip (src_imports
 // filters by the binding's own module — build+run exit 2). The
 // carrier's neg_selfimp leg is DROPPED here: