evolve: move extension metadata in their own module This will allow the server only extension to reuse them. As a side effect we can now simplify the config parsing in setup.py

evolve: specify the 'minimalhgversion' 3.7 added a new nice and shinny variable to help user know when they use the wrong version of Mercurial with an extension. We now use it.

serveronly: give the sub extension a way to access to the 'evolve' module We keep it as clean as possible but if the extension is specified with a direct path, we have to be a bit hacky. Being able to access the whole evolve extension from the 'serveronly' extension will lift multiple constraints on how we organise the code and will allow for cleaner and clearer code. We extract a minor function into a 'utility' module to have something to depends on.