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.

exthelper: add a 'merge' method This will help us spread more code in different sub module.

exthelper: extract into its own submodule That code is fairly independant, we extract it into its own extensions to clarify the code

makefile: add an 'install-home' target It is similar to the Mercurial one.

makefile: only check for HGROOT when running tests Otherwise we breaks other target that do not needs HGROOT.

tests: split pyflake and flake8 tests This allow more parallelism and better skip message

flake8: run on all python file We can now run it on all file in the repository to catch potential regression.