gpg-migrate.py: Convert PGPPacket 'packet-tag' key to 'type'

gpg-migrate.py: Convert PGPPacket 'packet-tag' key to 'type'

And convert the integer to a string using the table from RFC 4880 [1]:

  The packet tag denotes what type of packet the body holds.  Note
  that old format headers can only have tags less than 16, whereas new
  format headers can have tags as great as 63.  The defined tags (in
  decimal) are as follows:

       0        -- Reserved - a packet tag MUST NOT have this value
       1        -- Public-Key Encrypted Session Key Packet
       2        -- Signature Packet
       3        -- Symmetric-Key Encrypted Session Key Packet
       4        -- One-Pass Signature Packet
       5        -- Secret-Key Packet
       6        -- Public-Key Packet
       7        -- Secret-Subkey Packet
       8        -- Compressed Data Packet
       9        -- Symmetrically Encrypted Data Packet
       10       -- Marker Packet
       11       -- Literal Data Packet
       12       -- Trust Packet
       13       -- User ID Packet
       14       -- Public-Subkey Packet
       17       -- User Attribute Packet
       18       -- Sym. Encrypted and Integrity Protected Data Packet
       19       -- Modification Detection Code Packet
       60 to 63 -- Private or Experimental Values

[1]: http://tools.ietf.org/search/rfc4880#section-4.3