discovery: split discovery related code in 'obsdiscovery' More code splitting for more clarity.

exchange: rename the module Let's add a 'obs' prefix to make the module role clearer.

setup.py: include all evolve submodule We are splitting more and more code into submodule, we make sure their are properly installed. As a side effect the 'legacy' module is now installed, that is not too important. The 'evolve.hack' package is still non installed by default, as intended.

evolve: switch away from deprecated repo.opener

serveronly: use the same extensions metadata than evolve This will avoid them to drift out of sync with evolve.

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.

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.