Lint and refactor check-mirrors

Originally created by @intrigeri on #17704 (Redmine)

In its 8.5 years of existence, check-mirrors.rb has grown a lot — some would say “organically” — without us taking much time to refactor the code along the way. IMO this program it has become too long, poorly structured, hard to understand, to maintain, and to modify.

Additionally, the code style is quite inconsistent both internally and wrt. Ruby best practices.

This issue tracks a first iteration about these 2 technical debt problems, not aiming at perfection whatsoever, but instead with the goal to handle a first batch of low hanging fruits, and give ourselves a better starting point next time we work on this code base.

Feature Branch: lint