cache: move to backend package

internal/backend/cache/dir.go

@@ -0,0 +1,28 @@
+package cache
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// EnvDir return $RESTIC_CACHE_DIR env
+func EnvDir() string {
+	return os.Getenv("RESTIC_CACHE_DIR")
+}
+
+// DefaultDir returns $RESTIC_CACHE_DIR, or the default cache directory
+// for the current OS if that variable is not set.
+func DefaultDir() (cachedir string, err error) {
+	cachedir = EnvDir()
+	if cachedir != "" {
+		return cachedir, nil
+	}
+
+	cachedir, err = os.UserCacheDir()
+	if err != nil {
+		return "", fmt.Errorf("unable to locate cache directory: %v", err)
+	}
+
+	return filepath.Join(cachedir, "restic"), nil
+}