Create a MessageId class

[Pierre-Sassoulas]

Description

This create a MessageId class with its unit tests, and permit some simplification in pylint.message. It rename MessageStore to MessageDefinitionStore because we anticipate to have a MessageIdStore later. It will permit other simplifications in the next step of this refactor, but let's keep the MR small.

Type of Changes

	Type
✓	🔨 Refactoring

Related Issue

Next step in order to make #2992 easier to review.

[coveralls]

Coverage increased (+0.02%) to 89.993% when pulling edd7067 on Pierre-Sassoulas:create-a-pylint-message-id-class into 3b18105 on PyCQA:master.

[coveralls]

Coverage increased (+0.02%) to 89.993% when pulling edd7067 on Pierre-Sassoulas:create-a-pylint-message-id-class into 3b18105 on PyCQA:master.

[coveralls]

Coverage increased (+0.02%) to 89.993% when pulling edd7067 on Pierre-Sassoulas:create-a-pylint-message-id-class into 3b18105 on PyCQA:master.

[coveralls]

Coverage increased (+0.02%) to 89.993% when pulling edd7067 on Pierre-Sassoulas:create-a-pylint-message-id-class into 3b18105 on PyCQA:master.

[Pierre-Sassoulas]

Performance-wise this MessageId class was not a good idea. I'm removing it.