internal/http3: new package

Create a package for an HTTP/3 implementation.
Internal for now, intended to eventually move to x/net/http3.

For golang/go#70914

Change-Id: I3a643fe7958cf75b231ca97f25e9f338554f723c
Reviewed-on: https://go-review.googlesource.com/c/net/+/641836
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/http3/doc.go

@@ -0,0 +1,10 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package http3 implements the HTTP/3 protocol.
+//
+// This package is a work in progress.
+// It is not ready for production usage.
+// Its API is subject to change without notice.
+package http3

internal/http3/files_test.go

@@ -0,0 +1,56 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.24
+
+package http3
+
+import (
+	"bytes"
+	"os"
+	"strings"
+	"testing"
+)
+
+// TestFiles checks that every file in this package has a build constraint on Go 1.24.
+//
+// Package tests rely on testing/synctest, added as an experiment in Go 1.24.
+// When moving internal/http3 to an importable location, we can decide whether
+// to relax the constraint for non-test files.
+//
+// Drop this test when the x/net go.mod depends on 1.24 or newer.
+func TestFiles(t *testing.T) {
+	f, err := os.Open(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	names, err := f.Readdirnames(-1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range names {
+		if !strings.HasSuffix(name, ".go") {
+			continue
+		}
+		b, err := os.ReadFile(name)
+		if err != nil {
+			t.Fatal(err)
+		}
+		// Check for copyright header while we're in here.
+		if !bytes.Contains(b, []byte("The Go Authors.")) {
+			t.Errorf("%v: missing copyright", name)
+		}
+		// doc.go doesn't need a build constraint.
+		if name == "doc.go" {
+			continue
+		}
+		if !bytes.Contains(b, []byte("//go:build go1.24")) {
+			t.Errorf("%v: missing constraint on go1.24", name)
+		}
+		if bytes.Contains(b, []byte(`"testing/synctest"`)) &&
+			!bytes.Contains(b, []byte("//go:build go1.24 && goexperiment.synctest")) {
+			t.Errorf("%v: missing constraint on go1.24 && goexperiment.synctest", name)
+		}
+	}
+}