Support a "first version wins" semantics.

--- ./scripts/perl.prov.orig	2010-12-03 12:11:57.000000000 +0000
+++ ./scripts/perl.prov	2011-05-11 15:32:39.000000000 +0000
@@ -170,7 +170,7 @@ sub process_file {
 
         $version = $1;
       }
-      $require{$package} = $version;
+      $require{$package} ||= $version;
     }
 
     # Allow someone to have a variable that defines virtual packages