scmutil: add join method to opener to construct path relative to base
Useful when we only have the opener without the base path used when it was
constructed.
filecache: refactor path join logic to a function
New users of filecache use different names for the function used to compute
the runtime path of the cached file.
Users should subclass filecache and provide their own version of this
function to call the appropriate join function on 'obj' (an instance
of the class that its member function was decorated).