crypto/ed25519/internal/edwards25519: move to crypto/internal/edwards25519

crypto/ed25519/internal/edwards25519: move to crypto/internal/edwards25519

This will allow us to use crypto/internal/edwards25519/field from
crypto/ecdh to implement X25519, dropping the dependency on
golang.org/x/crypto/curve25519.

For #52182

Change-Id: I3be9debc6e13bf06944b98668f34313a975914d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/402556
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>