Disallow (and rewrite) injected variables

[MarkusTeufelberger]

See also #3456

Ansible since version 2.5 stores all its host facts in the ansible_facts dictionary, but since then has kept its previous design of injecting them into the global namespace. This is turned on by default (e.g. ansible_facts['distribution'] is also available as ansible_distribution).

This apparently has performance implications (lots of copying around facts for every single host in the inventory probably adds up over time), makes roles slightly less portable (in theory https://docs.ansible.com/ansible/latest/reference_appendices/config.html#inject-facts-as-vars could be turned off by users from its current default of being enabled already now) and there are apparently plans to change the default value and set INJECT_FACTS_AS_VARS=False as a default some time in the future (2.16 might already warn about this, the real change probably would end up in 2.20 or so).

This is also a great candidate for automatic transforms (ansible_distribution --> ansible_facts['distribution']) to make the transition easier or even automated for users. Either by being very explicit and safe (maintain a growing list of facts before/after) or by rewriting as much as possible (variable starts with ansible_...? rewrite to ansible_facts['...']!) from the get-go.

In any case, this is likely something that will come up this summer either way. It can also be tested already by users by setting fact injection to false to find out how/where they are affected.