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()`.