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.

test: update test to check for various way to import the serveronly things We add a "setup script" that update the PYTHONPATH to allow direct import of the extension code. Then we tests that all way to import the extensions works.

tests: move "test setup" script into a 'testlib' directory This will avoid cluttering the tests directory.

exchange: move code related to exchange into a 'evolve.exchange' submodule The evolve extension is HUGE, we split exchange code appart before doing more work on it.

exthelper: move command declaration into the exthelper This is necessary to allow sub module to declare commands too.