udev: derive path ID for PNP devices from ACPI firmware node

PNP devices may represent ACPI-enumerated hardware but do not have a
parent type supported by path_id. Consequently, importing path_id fails
even when the PNP device exposes a stable ACPI firmware_node. This also
prevents later assignments in rules such as the systemd-backlight
activation rule from taking effect.

Resolve the PNP device's firmware_node and use its ACPI sysname for the
path component. This gives PNP-backed devices the same stable identity
as their firmware representation.

Add a regression test using an RTC device below a PNP parent with an
ACPI firmware node.

test/sys-script.py

@@ -10584,6 +10584,7 @@ f('sys/devices/pnp0/00:07/rtc/rtc0/name', 0o644, b'rtc_cmos\n')
 f('sys/devices/pnp0/00:07/rtc/rtc0/time', 0o644, b'02:30:51\n')
 f('sys/devices/pnp0/00:07/rtc/rtc0/uevent', 0o644, b'''MAJOR=253
 MINOR=0
+DEVNAME=rtc0
 ''')
 d('sys/devices/pnp0/00:07/rtc/rtc0/power', 0o755)
 f('sys/devices/pnp0/00:07/rtc/rtc0/power/wakeup', 0o644, b'\n')

test/test-udev.py

@@ -2071,6 +2071,17 @@ SUBSYSTEMS=="scsi", PROGRAM=="/bin/bash -c \"printf %%s 'foo1 foo2' | grep 'foo1
         KERNEL=="sda", ENV{ID_PATH}=="?*", SYMLINK+="disk/by-path/$env{ID_PATH}"
         ''',
     ),
+    Rules.new(
+        'builtin path_id for PNP device with ACPI firmware node',
+        Device(
+            '/devices/pnp0/00:07/rtc/rtc0',
+            exp_links=['rtc/by-path/acpi-PNP0B00:00'],
+        ),
+        rules=r'''
+        KERNEL=="rtc0", IMPORT{builtin}="path_id"
+        KERNEL=="rtc0", ENV{ID_PATH}=="?*", SYMLINK+="rtc/by-path/$env{ID_PATH}"
+        ''',
+    ),
     Rules.new(
         'add and match tag',
         Device(