[linter] Linter for v2 migration

[jgsogo]

As we are moving closer to v2 we think a linter would be the perfect tool to help us during this process. Here I'm modifying existing linters and proposing a new path:

• linters will be compulsory, the PR will fail (and won't be merged) if any linter fail
• all the rules executed by the linter should be stored in CCI repository, we need this flexibility to add and remove these rules and checks following our own migration pace.
• Before adding a new rule, we need to check the number of recipes that will fail and how hard it will be to fix them. We might decide to postpone the addition if too many recipes are affected or the work required to fix them is too much at the moment.

Note. I will be adding an isolated yaml linter in the following hours

[ericLemanissier]

It's great. Don't you think we will have to add this https://github.com/conan-io/conan/blob/develop/conans/pylint_plugin.py#L10-L47 to the CCI linter anyway, in order to avoid "undefined variable" warning/errors ?

[jgsogo]

Sure, this is just the starting point: a very naïve linter that works for 100% of the recipes.

Next steps will be to evolve the linter and check ConanFile attributes, methods,... but it can be done in following PRs. As you can see, those following PRs will run against all recipes, so we can know before merging how many recipes are broken and decide.

[ericLemanissier]

ok, I had missed the disable=all, so these warnings won't show-up anyway. sorry!