The hpke package uses public-facing crypto packages except for hkdf,
which uses crypto/internal/hkdf. We already have a public hkdf package,
crypto/hkdf, so use it instead for consistency.
Change-Id: Icf6afde791234dfe24dbfba715c0891f32005ca2
Reviewed-on: https://go-review.googlesource.com/c/go/+/657556 Reviewed-by: Roland Shoemaker <roland@golang.org> Reviewed-by: Daniel McCarney <daniel@binaryparadox.net> Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>