Rename strutil to stringsx, use wg.Go

Address PR review feedback: rename the shared string-helper package
from strutil to stringsx to avoid the discouraged util suffix and the
collision with the standard strings package, updating all import paths
and call sites.

Replace the manual wg.Add/wg.Done bookkeeping in the enrichment worker
with wg.Go, which is less error-prone.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/thirdparty/common_third_party_owned_domains.go

@@ -18,7 +18,7 @@ import (
 	"slices"
 	"strings"
 
-	"go.probo.inc/probo/pkg/strutil"
+	"go.probo.inc/probo/pkg/stringsx"
 	"go.probo.inc/probo/pkg/uri"
 )
 
@@ -158,7 +158,7 @@ func vendorLabels(name, website string) []string {
 	}
 
 	add(uri.DomainLabel(website))
-	add(strutil.NormalizeAlnum(name))
+	add(stringsx.NormalizeAlnum(name))
 
 	return labels
 }