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>

handshake_server_tls13.go

@@ -22,7 +22,7 @@ import (
 	"slices"
 	"time"
 
-	"github.com/xtls/reality/mlkem768"
+	"github.com/xtls/reality/mlkem"
 	"github.com/xtls/reality/tls13"
 )
 
@@ -276,7 +276,7 @@ func (hs *serverHandshakeStateTLS13) processClientHello() error {
 	ecdhData := clientKeyShare.data
 	if selectedGroup == x25519Kyber768Draft00 {
 		ecdhGroup = X25519
-		if len(ecdhData) != x25519PublicKeySize+mlkem768.EncapsulationKeySize768 {
+		if len(ecdhData) != x25519PublicKeySize+mlkem.EncapsulationKeySize768 {
 			c.sendAlert(alertIllegalParameter)
 			return errors.New("tls: invalid Kyber client key share")
 		}