crypto/internal/mlkem768: move to crypto/internal/fips/mlkem

In the process, replace out-of-module imports with their FIPS versions.

For #69536

Change-Id: I83e900b7c38ecf760382e5dca7fd0b1eaa5a5589
Reviewed-on: https://go-review.googlesource.com/c/go/+/626879
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

randutil/randutil.go

@@ -0,0 +1,26 @@
+// Copyright 2018 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 randutil contains internal randomness utilities for various
+// crypto packages.
+package randutil
+
+import (
+	"io"
+	"math/rand/v2"
+)
+
+// MaybeReadByte reads a single byte from r with 50% probability. This is used
+// to ensure that callers do not depend on non-guaranteed behaviour, e.g.
+// assuming that rsa.GenerateKey is deterministic w.r.t. a given random stream.
+//
+// This does not affect tests that pass a stream of fixed bytes as the random
+// source (e.g. a zeroReader).
+func MaybeReadByte(r io.Reader) {
+	if rand.Uint64()&1 == 1 {
+		return
+	}
+	var buf [1]byte
+	r.Read(buf[:])
+}