ci: split the jobs on more stage
We start to have a lot of job, grouping them help to clarifying the pipeline.
We don't actually create dependency between each stage, so everything still run
concurrently. However we are about to introduce some wheel-building job that
will be reused by some tests. So some dependencies are coming.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).