Sign and encrypt include Public-Key Encrypted Session Key Packet for both signer and recipient?

Hi,

When signing and encrypting, why does both the webapp and keybase cmdline include Public-Key Encrypted Session Key Packet for both the signer and the recipient?

Check out the following command line:

```
$ keybase pgp encrypt max < "msg.txt" | pgpdump
New: Public-Key Encrypted Session Key Packet(tag 1)(524 bytes)
New version(3)
Key ID - 0x2478DF98CED355D3
Pub alg - RSA Encrypt or Sign(pub 1)
RSA m^e mod n(4096 bits) - ...
-> m = sym alg(1 byte) + checksum(2 bytes) + PKCS-1 block type 02
New: Public-Key Encrypted Session Key Packet(tag 1)(268 bytes)
New version(3)
Key ID - 0x980A3F0D01FE04DF
Pub alg - RSA Encrypt or Sign(pub 1)
RSA m^e mod n(2048 bits) - ...
-> m = sym alg(1 byte) + checksum(2 bytes) + PKCS-1 block type 02
New: Symmetrically Encrypted and MDC Packet(tag 18)(1 bytes) partial start
Ver 1
Encrypted data [sym alg is specified in pub-key encrypted session key]
```

Compare the above with the following:

```
$ gpg --encrypt --sign --armor -u matt@implbits.com -r themax@gmail.com < msg.txt | pgpdump
Old: Public-Key Encrypted Session Key Packet(tag 1)(268 bytes)
New version(3)
Key ID - 0x980A3F0D01FE04DF
Pub alg - RSA Encrypt or Sign(pub 1)
RSA m^e mod n(2048 bits) - ...
-> m = sym alg(1 byte) + checksum(2 bytes) + PKCS-1 block type 02
New: Symmetrically Encrypted and MDC Packet(tag 18)(512 bytes) partial start
Ver 1
Encrypted data [sym alg is specified in pub-key encrypted session key]
(plain text + MDC SHA1(20 bytes))
```

This is an issue because with Keybase, signed and encrypted PGP Messages contain a clear text identifier for the signer (the Key ID in the Public-Key Encrypted Session Key Packet). This is very uncool for scenario where someone want to author a PGP Message and communicate it via a public forum without divulging that there is a relationship with the recipient.

Reproduce the reported output with `keybase pgp encrypt` and `pgpdump`, then compare it with the `gpg --encrypt --sign` output shown in the issue. The payload names no source files or tests; done would require an agreed design and verification that signed, encrypted messages no longer expose the signer identifier while remaining valid.

Written by the indexing model from the issue text.