internal/subtle: rename to internal/alias

This avoids an import conflict in code that needs to import
crypto/subtle as well.

CL 424194 does the same for the main repo.

Change-Id: Ic54cb62bbfdcf5c2cb6f15ac47075ee1c41981ad
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/424175
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>

salsa20/salsa20.go

@@ -24,7 +24,7 @@ package salsa20 // import "golang.org/x/crypto/salsa20"
 // TODO(agl): implement XORKeyStream12 and XORKeyStream8 - the reduced round variants of Salsa20.
 
 import (
-	"golang.org/x/crypto/internal/subtle"
+	"golang.org/x/crypto/internal/alias"
 	"golang.org/x/crypto/salsa20/salsa"
 )
 
@@ -35,7 +35,7 @@ func XORKeyStream(out, in []byte, nonce []byte, key *[32]byte) {
 	if len(out) < len(in) {
 		panic("salsa20: output smaller than input")
 	}
-	if subtle.InexactOverlap(out[:len(in)], in) {
+	if alias.InexactOverlap(out[:len(in)], in) {
 		panic("salsa20: invalid buffer overlap")
 	}