[backend] tweak last commit so that it matches the rest of the code

I.e. do not mess with the "detect multibuild" regexp that is
used in other places as well. No functional changes.

src/backend/BSVerify.pm

@@ -43,9 +43,10 @@ sub verify_projkind {
 sub verify_packid {
   my $packid = $_[0];
   die("packid is empty\n") unless defined($packid) && $packid ne '';
-  if ($packid =~ /(?<!^_product)(?<!^_patchinfo)(?<!^_project)(?<!^_pattern):./) {
+  if ($packid =~ /(?<!^_product)(?<!^_patchinfo):./) {
     die("packid '$packid' is illegal\n") unless $packid =~ /^([^:]+):([^:]+)$/s;
     my ($p1, $p2) = ($1, $2);
+    die("packid '$packid' is illegal\n") if $p1 eq '_project' || $p1 eq '_pattern';
     verify_packid($p1);
     verify_packid($p2);
     return;