Register sync DBs manually to fix dyalpm returning empty list

The dyalpm library's SyncDBs() returns an empty slice, causing packages
from official repos (extra, core, multilib) to not be found. This fix
manually registers each repo after getting an empty result.

pkg/pacman/pacman.go

@@ -50,6 +50,14 @@ func New() (*Pac, error) {
 		return nil, fmt.Errorf("failed to get sync databases: %w", err)
 	}
 
+	if len(syncDBs) == 0 {
+		syncDBs, err = registerSyncDBs(handle)
+		if err != nil {
+			handle.Release()
+			return nil, fmt.Errorf("failed to register sync databases: %w", err)
+		}
+	}
+
 	fmt.Fprintf(os.Stderr, "[debug] New: done (%.2fs)\n", time.Since(start).Seconds())
 	return &Pac{
 		aurCache: make(map[string]AURPackage),
@@ -66,6 +74,33 @@ func (p *Pac) Close() error {
 	return nil
 }
 
+func registerSyncDBs(handle dyalpm.Handle) ([]dyalpm.Database, error) {
+	fmt.Fprintf(os.Stderr, "[debug] registerSyncDBs: starting...\n")
+
+	repos := []string{"core", "extra", "multilib"}
+	var dbs []dyalpm.Database
+
+	for _, repo := range repos {
+		db, err := handle.RegisterSyncDB(repo, 0)
+		if err != nil {
+			continue
+		}
+
+		count := 0
+		db.PkgCache().ForEach(func(pkg dyalpm.Package) error {
+			count++
+			return nil
+		})
+
+		if count > 0 {
+			dbs = append(dbs, db)
+		}
+	}
+
+	fmt.Fprintf(os.Stderr, "[debug] registerSyncDBs: done (%d dbs)\n", len(dbs))
+	return dbs, nil
+}
+
 type PackageInfo struct {
 	Name      string
 	InAUR     bool