internal/musig2: rename to musig2v040 to avoid confusion

Since we explicitly keep an old version of a library in lnd for backward
compatibility we want to make sure the purpose and version of it is
clear and not misleading.

internal/musig2v040/README.md

@@ -0,0 +1,10 @@
+# MuSig2 v0.4.0
+
+This package contains an exact copy of the MuSig2 code as found in
+`github.com/btcsuite/btcec/v2/schnorr/musig2` at the tag `btcec/v2.2.2`.
+
+This corresponds to the [MuSig2 BIP specification version of
+`v0.4.0`](https://github.com/jonasnick/bips/blob/musig2/bip-musig2.mediawiki).
+
+We only keep this code here to allow implementing a backward compatible,
+versioned MuSig2 RPC.

internal/musig2v040/bench_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
-package musig2
+package musig2v040
 
 import (
 	"encoding/hex"

internal/musig2v040/context.go

@@ -1,6 +1,6 @@
 // Copyright (c) 2013-2022 The btcsuite developers
 
-package musig2
+package musig2v040
 
 import (
 	"fmt"

internal/musig2v040/keys.go

@@ -1,6 +1,6 @@
 // Copyright 2013-2022 The btcsuite developers
 
-package musig2
+package musig2v040
 
 import (
 	"bytes"

internal/musig2v040/musig2_test.go

@@ -1,6 +1,6 @@
 // Copyright 2013-2022 The btcsuite developers
 
-package musig2
+package musig2v040
 
 import (
 	"bytes"

internal/musig2v040/nonces.go

@@ -1,6 +1,6 @@
 // Copyright 2013-2022 The btcsuite developers
 
-package musig2
+package musig2v040
 
 import (
 	"bytes"

internal/musig2v040/sign.go

@@ -1,6 +1,6 @@
 // Copyright 2013-2022 The btcsuite developers
 
-package musig2
+package musig2v040
 
 import (
 	"bytes"