shared/conf-parser: collapse pkgdir and conf_file args into one

This essentially reverts 5656cdfeea. I find it
much easier to understand what is going on when the
path-relative-to-the-search-path is passed in full, instead of being constructed
from two parts, with one of the parts being implicit in some places.

Also, we call 'systemd-analyze cat-config <path>' with <path> with the same
meaning, so this makes the internal and external APIs more consistent.

src/shared/udev-util.c

@@ -28,9 +28,8 @@ int udev_parse_config_full(const ConfigTableItem config_table[]) {
 
         assert(config_table);
 
-        r = config_parse_config_file_full(
-                        "udev.conf",
-                        "udev",
+        r = config_parse_config_file(
+                        "udev/udev.conf",
                         /* sections = */ NULL,
                         config_item_table_lookup,
                         config_table,