core: change top-level drop-in from -.service.d to service.d

Discussed in #13743, the -.service semantic conflicts with the
existing root mount and slice names, making this feature not
uniformly extensible to all types. Change the name to be
<type>.d instead.

Updating to this format also extends the top-level dropin to
unit types.

src/basic/unit-name.c

@@ -669,36 +669,6 @@ good:
         return 0;
 }
 
-bool service_unit_name_is_valid(const char *name) {
-        _cleanup_free_ char *prefix = NULL, *s = NULL;
-        const char *e, *service_name = name;
-
-        if (!unit_name_is_valid(name, UNIT_NAME_ANY))
-                return false;
-
-        e = endswith(name, ".service");
-        if (!e)
-                return false;
-
-        /* If it's a template or instance, get the prefix as a service name. */
-        if (unit_name_is_valid(name, UNIT_NAME_INSTANCE|UNIT_NAME_TEMPLATE)) {
-                if (unit_name_to_prefix(name, &prefix) < 0)
-                        return false;
-
-                s = strjoin(prefix, ".service");
-                if (!s)
-                        return false;
-
-                service_name = s;
-        }
-
-        /* Reject reserved service name(s). */
-        if (streq(service_name, SPECIAL_ROOT_SERVICE))
-                return false;
-
-        return true;
-}
-
 int slice_build_parent_slice(const char *slice, char **ret) {
         char *s, *dash;
         int r;