I think we can actually use a `packetlist` here because we have that check
```
if (pkESKeyPacket.publicKeyId.equals(privateKeyPacket.getKeyId())) {
```
we should be able to get the key from the list with the `getKeyPacketsById`.  I think this is worthwhile because often times a consumer of our library will know they have a key that something was sent for but would require openpgp.js to know which of the subkeys that would be.  Thoughts?