Simplify `External` and `Loadable` usage

If we're not using the `External::Loaded` variant at all, the whole type should probably away. The sole purpose of the `enum` was to make it easy to supply custom values for configuration that may be in an external file in a test (temporary files have to be used instead if this is removed). Currently this functionality is not used at all, so maybe it is time to go.

We probably want to go a step further and remove the logic from the configuration entirely. Components would then themselves be responsible for loading data from a file. This does move the point of failure back a bit (from configuration load to execution), but potentially the reduced complexity is worth it.