lister: Allow lister to build url out of the instance parameter

This pushes the rather elementary logic within the lister's scope. This will simplify
and unify cli call between lister and scheduler clis. This will also allow to reduce
erroneous operations which can happen for example in the add-forge-now.

With the following, we will only have to provide the type and the instance, then
everything will be scheduled properly.

Refs. swh/devel/swh-lister#4693