feat: get_service_packages, which packages a systemd service is (NetOrk/netork#1236)

Mixes in napalm-device-types' ServicePackagesMixin. The transport runs the
shared read-only command on an exec channel under sh, without sudo: it is
about 1 kB, past what the PTY takes reliably, and written for sh whatever
the login shell. Requires napalm-device-types 4.20.0; version 0.12.0.

tests/test_linux.py

@@ -1193,6 +1193,40 @@ def test_get_kernel_facts_raises_on_output_without_a_report(driver):
             driver.get_kernel_facts()
 
 
+# ---------------------------------------------------------------------------
+# get_service_packages -- the command and its parse live in napalm-device-types
+# ---------------------------------------------------------------------------
+
+_SVCPKG = (
+    "SVCPKG_BEGIN\n[units]\nId=nginx.service\n"
+    "FragmentPath=/lib/systemd/system/nginx.service\n"
+    "ExecStart={ path=/usr/sbin/nginx ; argv[]=/usr/sbin/nginx }\n\n"
+    "[dpkg]\nnginx-common: /lib/systemd/system/nginx.service\n"
+    "nginx-core: /usr/sbin/nginx\nSVCPKG_END\n"
+)
+
+
+def test_get_service_packages_runs_the_shared_command_under_sh_on_an_exec_channel(driver):
+    """Read-only, so never through sudo; ``sh`` because the login shell may be another."""
+    from shlex import quote
+
+    from napalm_device_types import CommandResult, ServicePackagesMixin
+    from napalm_device_types.service_packages import SERVICE_PACKAGES_COMMAND
+
+    assert isinstance(driver, ServicePackagesMixin)
+    calls = []
+
+    def run_command(command, *, privileged=False, timeout=60, stdin=None):
+        calls.append((command, privileged))
+        return CommandResult(_SVCPKG, "", 0)
+
+    driver.run_command = run_command
+
+    assert driver.get_service_packages() == {"nginx": ["nginx-common", "nginx-core"]}
+    assert calls == [(f"sh -c {quote(SERVICE_PACKAGES_COMMAND)}", False)]
+    driver._device.send_command.assert_not_called()
+
+
 # ---------------------------------------------------------------------------
 # Services: listed in one round trip, controlled through systemctl (#7)
 # ---------------------------------------------------------------------------