Add missing `require`

[yogeshjain999]

Issue

reform is failing on requiring latest changes from master because of some missing requires.

Test to reproduce

https://github.com/yogeshjain999/reform/commit/cfb58e3f1ad30f992778bb2d0d23d9fb8ea2d10d

Test run

https://github.com/yogeshjain999/reform/actions/runs/732842422

Fix

Make requiring files (xml, json, hash etc) explicitly possible. This includes some reverts from PR #235 and commit 411abe72. Reasons,

• Many files like representable/json, representable/xml etc are loaded directly without require representable within dependencies.
• Some dependencies (eg: reform) directly refers constants which are marked to autoload (eg: Representable::Decorator).