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>

chacha20/chacha_generic.go

@@ -12,7 +12,7 @@ import (
 	"errors"
 	"math/bits"
 
-	"golang.org/x/crypto/internal/subtle"
+	"golang.org/x/crypto/internal/alias"
 )
 
 const (
@@ -189,7 +189,7 @@ func (s *Cipher) XORKeyStream(dst, src []byte) {
 		panic("chacha20: output smaller than input")
 	}
 	dst = dst[:len(src)]
-	if subtle.InexactOverlap(dst, src) {
+	if alias.InexactOverlap(dst, src) {
 		panic("chacha20: invalid buffer overlap")
 	}