Allow downstreams to enforce that the ansible_core ref must have a version specifier

[sivel]

This PR allows downstreams to patch REQUIRE_ANSIBLE_CORE_PIN in constants.py to a value of True to force EE definitions to contain a pin for the ansible-core specification.

This would help ensure that users building custom EEs would have to put thought into the version of ansible-core that they intend to be using, without unexpected upgrades in the building process pulling in a newer version they have not tested against.

No evaluation of the pin is done, this is a minor safety net.

[Shrews]

I'd much rather see this validation done in the UserDefinition.validate() method where everything else is validated.

[sivel]

I'd much rather see this validation done in the UserDefinition.validate() method where everything else is validated

Done.

[Shrews]

Suggested an additional test for complete code coverage, and a spelling correction. Otherwise, LGTM.

[sivel]

I've accepted the review changes, looks like there is at minimum a linting issue which I'll resolve though. I just want to note that the resolution of these things doesn't mean this needs merged.

This is an idea, which we ultimately need to decide whether we want it or not.

[Shrews]

LGTM, but not merging until we decide we actually want this.