Custom repository types are registered through the RepositoriesModule.
Later, when a specific repository type is used, the RespositoryModule
is installed, which in turn would spawn the module that was
registered for that repository type. However, a module is not needed
here. Each repository type has two associated classes, a Repository and
an IndexShardRepository.

This change makes the registration method for custom repository
types take both of these classes, instead of a module.

See elastic#12783.