Allow streams version to be parameterised so that it can be different for tools and images

This started out as a branch to use "v2" simplestreams metadata for tools to allow different metadata for 1.20 and newer vs 1.18 and older version of Juju. The fix is to change how simplestreams paths are resolved such that the streams version is now parameterised.

A different direction was decided on, but this change is still useful because it simplifies how Fetch() is invoked, provided a cleaner implementation of path resolution, and allows for easy changes of the streams versions in the future when needed. The changes are mechanical.