Move loaders to CSET.loaders

[jfrost-mo]

While writing the architecture documentation it became apparent that the loaders shouldn't be under the recipes, and should instead be their own top level directory.

By doing this now we can avoid the pain of people having to know of the change of location.

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• Documentation has been updated to reflect change. (I've updated the architecture PR.)
• New code has tests, and affected old tests have been updated.
• All tests and CI checks pass.
• Ensured the pull request title is descriptive.
• Conda lock files have been updated if dependencies have changed.
• Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• Marked the PR as ready to review.

[github-actions]