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