[CLEANUP BUGFIX] fix model and factory lookup

[runspired]

Resolves #4711
Resolves an issue with createRecord failing when being called for a non-existent model
Fixes an issue found with toString implementation in #5117

Example Fail case prior to this PR:

let person = run(() => store.push({
    data: {
      type: 'person',
      id: '1'
    }
}));

person.constructor.modelName; // will be unset

Previously, we had overly complicated the process of looking up a model with a mixture of public and private and public-looking but private methods. This complexity resulted in there being multiple code-paths by which a ModelClass could be looked up without a modelName or in which no factory for the ModelClass would be found but we would still attempt to create an instance from a missing factory.

Much of this complexity stemmed from the desire to check whether a ModelClass existed. I have strengthened our private _hasModelFor implementation, and trimmed us down to a single public modelFor hook and a single private _modelFactoryFor hook to eliminate confusion and prevent these sorts of errors from occurring.

Deprecates

• _modelForMixin
• _modelFor
• modelFactoryFor

[runspired]

cc @samselikoff @DingoEatingFuzz

[runspired]

cc @Exelord for ember-custom-actions

This: https://github.com/Exelord/ember-custom-actions/blob/9c3f89723bb54a51a6bf480b866e8b458bf7ba54/addon/serializers/rest.js#L20

Will become this: store._hasModelFor(modelName).

This is the only usage found in the wild by ember-observer code-search for the now deprecated version of modelFactoryFor (besides ember-m3 which I've opened an issue for).

Happily no addons use _modelFor or _modelForMixin

[runspired]

@Exelord I also meant to tell you that _hasModelFor is backwards compatible and is what should already have been in use there, it's safe to preemptively refactor to that now :)

[Exelord]

Ok! Thanks a lot Chris! I already migrated to it in the local branch. I will update the package soon :)

[bmac]

👍 Great work @runspired.