packer: partially restore buildPackagesFromFlags()

Commit a59c9ad was not entirely correct:
It is desired to call getGokrazySystemPackages(), but we should also
keep looking at cfg.Packages (which the commit didn’t).

internal/packer/packer.go

@@ -92,6 +92,13 @@ func buildPackageMapFromFlags(cfg *config.Struct) map[string]bool {
 	return buildPackages
 }
 
+func buildPackagesFromFlags(cfg *config.Struct) []string {
+	var buildPackages []string
+	buildPackages = append(buildPackages, cfg.Packages...)
+	buildPackages = append(buildPackages, getGokrazySystemPackages(cfg)...)
+	return buildPackages
+}
+
 func findFlagFiles(cfg *config.Struct) (map[string][]string, error) {
 	if len(cfg.PackageConfig) > 0 {
 		contents := make(map[string][]string)
@@ -625,7 +632,7 @@ func FindExtraFiles(cfg *config.Struct) (map[string][]*FileInfo, error) {
 		// fall through to look for extra files in <pkg>/_gokrazy/extrafiles
 	}
 
-	buildPackages := getGokrazySystemPackages(cfg)
+	buildPackages := buildPackagesFromFlags(cfg)
 	packageDirs, err := packer.PackageDirs(buildPackages)
 	if err != nil {
 		return nil, err