cmd/pkgserver: refer to preset in index

This enables referencing back to internal/rosa through an entry obtained via the index.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/pkgserver/index.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"slices"
 	"strings"
 	"unique"
@@ -26,6 +27,7 @@ type packageIndex struct {
 
 // metadata holds [rosa.Metadata] extended with additional information.
 type metadata struct {
+	p rosa.PArtifact
 	*rosa.Metadata
 
 	// Populated via [rosa.Toolchain.Version], [rosa.Unversioned] is equivalent
@@ -50,9 +52,17 @@ func (index *packageIndex) populate(cache *pkg.Cache, report *rosa.Report) (err
 	index.names = make(map[string]*metadata)
 	for p := range rosa.PresetUnexportedStart {
 		m := metadata{
+			p: p,
+
 			Metadata: rosa.GetMetadata(p),
 			Version:  rosa.Std.Version(p),
 		}
+		if m.Version == "" {
+			return errors.New("invalid version from " + m.Name)
+		}
+		if m.Version == rosa.Unversioned {
+			m.Version = ""
+		}
 
 		if cache != nil && report != nil {
 			m.ident = cache.Ident(rosa.Std.Load(p))