Add Go package registry (#24687)

Fixes #7608

This PR adds a Go package registry usable with the Go proxy protocol.

![grafik](https://github.com/go-gitea/gitea/assets/1666336/328feb5c-3df2-4f9d-8eae-fe3126d14c37)

modules/packages/goproxy/metadata.go

@@ -0,0 +1,94 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package goproxy
+
+import (
+	"archive/zip"
+	"fmt"
+	"io"
+	"path"
+	"strings"
+
+	"code.gitea.io/gitea/modules/util"
+)
+
+const (
+	PropertyGoMod = "go.mod"
+
+	maxGoModFileSize = 16 * 1024 * 1024 // https://go.dev/ref/mod#zip-path-size-constraints
+)
+
+var (
+	ErrInvalidStructure  = util.NewInvalidArgumentErrorf("package has invalid structure")
+	ErrGoModFileTooLarge = util.NewInvalidArgumentErrorf("go.mod file is too large")
+)
+
+type Package struct {
+	Name    string
+	Version string
+	GoMod   string
+}
+
+// ParsePackage parses the Go package file
+// https://go.dev/ref/mod#zip-files
+func ParsePackage(r io.ReaderAt, size int64) (*Package, error) {
+	archive, err := zip.NewReader(r, size)
+	if err != nil {
+		return nil, err
+	}
+
+	var p *Package
+
+	for _, file := range archive.File {
+		nameAndVersion := path.Dir(file.Name)
+
+		parts := strings.SplitN(nameAndVersion, "@", 2)
+		if len(parts) != 2 {
+			continue
+		}
+
+		versionParts := strings.SplitN(parts[1], "/", 2)
+
+		if p == nil {
+			p = &Package{
+				Name:    strings.TrimSuffix(nameAndVersion, "@"+parts[1]),
+				Version: versionParts[0],
+			}
+		}
+
+		if len(versionParts) > 1 {
+			// files are expected in the "root" folder
+			continue
+		}
+
+		if path.Base(file.Name) == "go.mod" {
+			if file.UncompressedSize64 > maxGoModFileSize {
+				return nil, ErrGoModFileTooLarge
+			}
+
+			f, err := archive.Open(file.Name)
+			if err != nil {
+				return nil, err
+			}
+			defer f.Close()
+
+			bytes, err := io.ReadAll(&io.LimitedReader{R: f, N: maxGoModFileSize})
+			if err != nil {
+				return nil, err
+			}
+
+			p.GoMod = string(bytes)
+
+			return p, nil
+		}
+	}
+
+	if p == nil {
+		return nil, ErrInvalidStructure
+	}
+
+	p.GoMod = fmt.Sprintf("module %s", p.Name)
+
+	return p, nil
+}