back.rtlil: put hierarchy in module name instead of an attribute.

The attribute sees essentially no use and the information is much
better served by putting it in the module name. In addition this
means that the entire tree can be renamed simply by renaming the top
module.

Tools like GTKWave show the names of the instances, not the modules,
so they are not affected by the longer names.

amaranth/back/rtlil.py

@@ -820,11 +820,10 @@ def _convert_fragment(builder, fragment, name_map, hierarchy):
         else:
             return "\\{}".format(fragment.type), port_map
 
-    module_name  = hierarchy[-1] or "anonymous"
+    module_name  = ".".join(name or "anonymous" for name in hierarchy)
     module_attrs = OrderedDict()
     if len(hierarchy) == 1:
         module_attrs["top"] = 1
-    module_attrs["amaranth.hierarchy"] = ".".join(name or "anonymous" for name in hierarchy)
 
     with builder.module(module_name, attrs=module_attrs) as module:
         compiler_state = _ValueCompilerState(module)