Why:
These are essentially internal methods that should not be publicly
available from the base namespace.
One thing worth noticing is that the use of this methods internally was
almost exclusively in the `syntax/default` except for one use on the
`factory_bot/definition`.
Also the deprecation silencing module was referring to the singleton
instance of the ```ActiveRecord::Deprecation``` class and not to the new
deprecation instance that was being used in the ```FactoryBot``` module.
This PR:
- Moves the `trait_by_name` and `register_trait` into the
`FactoryBot::Internal` module
- Deprecates uses of `trait_by_name`, `register_trait` and `traits` from
the `FactoryBot` module.
- Rename DEPRECATOR => Deprecation
This is one of the steps towards fixing [this
issue](https://github.com/thoughtbot/factory_bot/issues/1281)
The `default:` option of `mattr_accessor` wasn't added until Rails 5.2,
which means it has no effect on older versions and the default value of
`FactoryBot.use_parent_strategy` in 5.0.0.rc1 is nil on those versions.
