Implement Loader factory

[odashi]

Fixes #421

This pr implements a factory to obtain Loader classes, and removes the loader registry.

[odashi]

@neubig This is the same situation that I/ @tetsuok / @pfliu-nlp discussed previously:

https://github.com/neulab/ExplainaBoard/pull/434#issuecomment-1255500133

• Key type is not a pure str: This requires us to implement a factory regardless of whether we use the common registry or not.
• We don't need to serialize these classes.: This is the only remaining reason to use the common registry

At last, we eventually have no reason to adopt the common registry for Loader functionalities at this point.