feature(keyringctl): verify file structure integrity and packets

This moves all verify code to an own module and adds support to check
all packet files in the structure for integrity. This is done by parsing
assumptions like packet kind, type, issuer and location etc.

libkeyringctl/sequoia.py

@@ -11,6 +11,7 @@ from typing import List
 from typing import Optional
 
 from .types import Fingerprint
+from .types import PacketKind
 from .types import Uid
 from .types import Username
 from .util import cwd
@@ -208,6 +209,26 @@ def packet_signature_creation_time(packet: Path) -> datetime:
     return datetime.strptime(packet_dump_field(packet, "Signature creation time"), "%Y-%m-%d %H:%M:%S %Z")
 
 
+def packet_kinds(packet: Path) -> List[PacketKind]:
+    """Retrieve the PGP packet types of a packet path
+
+    Parameters
+    ----------
+    packet: The path to the PGP packet to retrieve the kind of
+
+    Returns
+    -------
+    The kind of PGP packet
+    """
+
+    dump = packet_dump(packet)
+    lines = [line for line in dump.splitlines()]
+    lines = list(
+        filter(lambda line: not line.startswith(" ") and not line.startswith("WARNING") and line.strip(), lines)
+    )
+    return [PacketKind(line.split()[0]) for line in lines]
+
+
 def latest_certification(certifications: Iterable[Path]) -> Path:
     """Returns the latest certification based on the signature creation time from a list of packets.