feat: get_service_packages, which packages a node's systemd unit is (NetOrk/netork#1236)

Mixes in napalm-device-types' ServicePackagesMixin; the transport is the
node's exec path, as for the kernel facts. Read-only. Requires
napalm-device-types 4.20.0; version 0.6.0.

tests/test_service_packages.py

@@ -0,0 +1,43 @@
+"""`get_service_packages`: which installed packages each systemd unit of the node is.
+
+The node's own services -- pveproxy, corosync, ceph -- are where a decision
+about a service lands on a hypervisor (NetOrk/netork#1236). The command and its
+parse are napalm-device-types'; the driver only carries the command over its
+exec path.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+
+from napalm_device_types import ServicePackagesMixin
+from napalm_device_types.service_packages import SERVICE_PACKAGES_COMMAND
+from napalm_proxmox.driver import ProxmoxDriver
+
+WIRE = (
+    "SVCPKG_BEGIN\n[units]\nId=pveproxy.service\n"
+    "FragmentPath=/lib/systemd/system/pveproxy.service\n"
+    "ExecStart={ path=/usr/bin/pveproxy ; argv[]=/usr/bin/pveproxy start }\n\n"
+    "[dpkg]\npve-manager: /lib/systemd/system/pveproxy.service\n"
+    "pve-manager: /usr/bin/pveproxy\nSVCPKG_END"
+)
+
+
+def test_the_driver_declares_the_contract():
+    assert issubclass(ProxmoxDriver, ServicePackagesMixin)
+
+
+def test_it_runs_the_shared_command(driver):
+    with patch.object(driver, "_exec_ssh_command", return_value=WIRE) as exec_:
+        found = driver.get_service_packages()
+
+    exec_.assert_called_once_with(SERVICE_PACKAGES_COMMAND)
+    assert found == {"pveproxy": ["pve-manager"]}
+
+
+def test_output_without_a_report_raises(driver):
+    with patch.object(driver, "_exec_ssh_command", return_value=""):
+        with pytest.raises(ValueError):
+            driver.get_service_packages()