Revert "Use import of OCI structs (#22765) (#22805)"

This reverts commit a239d6c4a9.

Refs: https://codeberg.org/forgejo/forgejo/issues/334

modules/packages/container/oci/digest.go

@@ -0,0 +1,27 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package oci
+
+import (
+	"regexp"
+	"strings"
+)
+
+var digestPattern = regexp.MustCompile(`\Asha256:[a-f0-9]{64}\z`)
+
+type Digest string
+
+// Validate checks if the digest has a valid SHA256 signature
+func (d Digest) Validate() bool {
+	return digestPattern.MatchString(string(d))
+}
+
+func (d Digest) Hash() string {
+	p := strings.SplitN(string(d), ":", 2)
+	if len(p) != 2 {
+		return ""
+	}
+	return p[1]
+}