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>

key_schedule_test.go

@@ -6,8 +6,8 @@ package tls
 
 import (
 	"bytes"
+	"crypto/internal/fips/mlkem"
 	"crypto/internal/fips/tls13"
-	"crypto/internal/mlkem768"
 	"crypto/sha256"
 	"encoding/hex"
 	"strings"
@@ -120,7 +120,7 @@ func TestTrafficKey(t *testing.T) {
 }
 
 func TestKyberEncapsulate(t *testing.T) {
-	dk, err := mlkem768.GenerateKey768()
+	dk, err := mlkem.GenerateKey768()
 	if err != nil {
 		t.Fatal(err)
 	}