The taglib directive has a tagdir attribute that is used to describe a directory
for the location of .tag files.
This seems a clean way to refer to a collection of actions without creating a
URI just to refer to them. The situation is quite messier for normal custom
actions, where we overload the semantics of the uri attribute.
Note that this overload is also breaking transparency: it is quite visible
whether the custom actions are implemented via a .tag file or via a custom
action. It is arguable whether that is good or not.