internal/chacha20: move up from chacha20poly1305/internal/chacha20

This exposes the chacha20 stream cipher to the entire x/crypto
package, and in particular to the SSH package, which uses separate,
unauthenticated chacha20 encryption for packet lengths.

Change-Id: I0b705482128f0657c09292370f03d08b588f7fec
Reviewed-on: https://go-review.googlesource.com/87075
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

chacha20poly1305/chacha20poly1305_generic.go

@@ -7,7 +7,7 @@ package chacha20poly1305
 import (
 	"encoding/binary"
 
-	"golang.org/x/crypto/chacha20poly1305/internal/chacha20"
+	"golang.org/x/crypto/internal/chacha20"
 	"golang.org/x/crypto/poly1305"
 )