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.h

@@ -58,8 +58,6 @@ static inline int unit_name_mangle(const char *name, UnitNameMangle flags, char
         return unit_name_mangle_with_suffix(name, NULL, flags, ".service", ret);
 }
 
-bool service_unit_name_is_valid(const char *name);
-
 int slice_build_parent_slice(const char *slice, char **ret);
 int slice_build_subslice(const char *slice, const char *name, char **subslice);
 bool slice_name_is_valid(const char *name);