Implement some code review suggestion

* Rename CLI flags

  * --no-digest-only
    Only prune images without a digest specified ("fallback" images usually)
  * --unreferenced-only
    Only prune downloads not referenced in any VM or template

* Replace sha256.sha256() call with `digest` module functions

Signed-off-by: Yury Bushmelev <jay4mail@gmail.com>

cmd/limactl/prune.go

@@ -1,15 +1,14 @@
 package main
 
 import (
-	"crypto/sha256"
-	"fmt"
 	"os"
 	"path/filepath"
 
 	"github.com/lima-vm/lima/pkg/downloader"
 	"github.com/lima-vm/lima/pkg/limayaml"
 	"github.com/lima-vm/lima/pkg/store"
 	"github.com/lima-vm/lima/pkg/templatestore"
+	"github.com/opencontainers/go-digest"
 	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 )
@@ -23,17 +22,17 @@ func newPruneCommand() *cobra.Command {
 		ValidArgsFunction: cobra.NoFileCompletions,
 	}
 
-	pruneCommand.Flags().BoolP("fallback", "F", false, "Only prune images without a digest specified (fallback images usually)")
-	pruneCommand.Flags().BoolP("unreferenced", "U", false, "Only prune downloads not referenced in any VM or template")
+	pruneCommand.Flags().Bool("no-digest-only", false, "Only prune images without a digest specified (\"fallback\" images usually)")
+	pruneCommand.Flags().Bool("unreferenced-only", false, "Only prune downloads not referenced in any VM or template")
 	return pruneCommand
 }
 
 func pruneAction(cmd *cobra.Command, args []string) error {
-	pruneFallback, err := cmd.Flags().GetBool("fallback")
+	pruneFallback, err := cmd.Flags().GetBool("no-digest-only")
 	if err != nil {
 		return err
 	}
-	pruneUnreferenced, err := cmd.Flags().GetBool("unreferenced")
+	pruneUnreferenced, err := cmd.Flags().GetBool("unreferenced-only")
 	if err != nil {
 		return err
 	}
@@ -90,10 +89,6 @@ func pruneAction(cmd *cobra.Command, args []string) error {
 	return os.RemoveAll(cacheDir)
 }
 
-func makeDigest(s string) string {
-	return fmt.Sprintf("%x", sha256.Sum256([]byte(s)))
-}
-
 // Collect all downloads referenced in VM definitions and templates
 func getReferencedDownloads() (map[string]limayaml.File, error) {
 	digests := make(map[string]limayaml.File)
@@ -110,8 +105,9 @@ func getReferencedDownloads() (map[string]limayaml.File, error) {
 
 	logrus.Debug("Referenced downloads:")
 	for _, f := range append(vmRefs, tmplRefs...) {
-		logrus.Debugf("+ %q => %q", makeDigest(f.Location), f)
-		digests[makeDigest(f.Location)] = f
+		d := digest.SHA256.FromString(f.Location).Encoded()
+		logrus.Debugf("+ %q => %q", d, f)
+		digests[d] = f
 	}
 	logrus.Debug(".")
 	return digests, nil