De-duplicate not needed certifications by cleaning the keyring after
import to remove old files when processing revocations. This basically
adds the functionality compared to import-clean.
libkeyringctl/keyring.py:
Simplify `convert_certificate()` by splitting out the conversion of
signature packets to `convert_signature_packet()` and the persistence of
packet material to `persist_key_material()`.
Add `convert_pubkey_signature_packet()`,
`convert_uid_signature_packet()` and
`convert_subkey_signature_packet()` to deal with the conversion of
public key signatures, UID signatures and subkey signatures
(respectively).
tests/test_keyring.py:
Add tests for `convert_certificate()`, `convert_signature_packet()`,
`convert_{pubkey,uid,subkey}_signature_packet()` and
`persist_subkey_revocations()`.
libkeyringctl/keyring.py:
Change `get_packets_from_path()` to use full conditional statements
which is easier to cover in tests.
tests/test_keyring.py:
Add simple tests for `get_packets_from_path()`,
`get_packets_from_listing()`, `export()` and `build()`.
tests/test_keyring.py:
Add tests for `is_pgp_fingerprint()`,
`transform_{fingerprint,username}_to_keyring_path()`,
`derive_username_from_fingerprint()` and get_fingerprints_from_paths()`.
