Base the Model Registry on constructors, rather than model types; and ancillary changes

[ablaom]

In this PR we:

• Provide a mechanism for including wrappers, such as TunedModel, in the Model Registry by adding a new option wrappers=false to models(...) method. The false fallback ensures no breakages for normal usage. In generating the model registry, use wrappers=true to ensure wrappers are now included.

• Change the model registry generation process so that name is the always name of a constructor rather than the name of a model type, to support Enable entry of model wrappers into the MLJ Model Registry MLJ.jl#1125. For regular models, there is no difference, but for wrappers (previously excluded from the registry) there is. For example, TunedModel is the constructor for ProbabilisticTunedModel and DeterministicTunedModel types, but while the constructor is user-facing, the types are not. So, in the updated registry, only TunedModel should appear.

• Add FeatureSelection.jl models to the Model Registry

• Remove MLJModels.FeatureSelector src code, as this model has been migrated to FeatureSelection.jl

• Update the ModelRegistry again to remove MLJModels.FeatureSelector from the registry

The last three items render #556 redundant (that PR has been merged into the current one)

This PR has been tested locally with a planned downstream update to MLJ.jl.

cc @OkonSamuel @EssamWisam