ww: select build action from package declaration

test/wcc/data/r949_xmod_fnptr_const_table/case.ww

@@ -1,4 +1,5 @@
 //ww:run
+package main;
 // migrated from test/wcc/949_xmod_fnptr_const_run.c: cross-module &cc.fn inside const table + scalar emits the &fn->DATAR reloc (#124); DISTINCT predicates catch a swapped reloc.
 package cc;
 export fn isa(c: rune) bool = { return c == 'a'; };