rules: Move the ensure_branch rules to a separate ruleset

The ensure_branch rules in the main rule file didn't really work out.

The original idea was to describe the desired status and so that branching was an idempotent operation.

Unfortunately, things are more complicated.

For instance, let's say we have a package in v2020dev1 and we want to drop it in v2020pre: we want to drop the v2020pre branch, but then the ensure_branch rule won't know it was removed on purpose and will try to recreate it.

Let's then drop the branch creation from the main ruleset as it is really meant to be runnable at any time, and then deal with branching in a separate ruleset that should be launched only at branching time to avoid ill effects.