(Maint) Removed commented out settings

These settings were commented out. I think we have already done
something else to support those use cases, and if we haven't...well,
that is what version control is for.
(Maint) Restructure requires for settings

The settings classes were not loaded in a robust manner. Some required
other files with the knowledge that the right modules would be created.
This caused a duplication of requires (which can cause a small slowdown
as ruby looks up whether the code has already been loaded). By requiring
the settings *after* the settings class has been opened, we can avoid a
lot of this extra work and overhead.
(#22205) Add enum setting type

It is often useful to be able to restrict the valid values for a setting
to a known set. This adds an :enum setting type that can take a :values
array, which defines the allowed values. If the value given for the
setting is now found in the list of allowed values, then it will throw
an error.
(#22205) Provide setting to control manifest ordering

The default ordering for manifests is something that is useful to be
able to control. This provides an setting that will be used to select
between the pre-#18508 and post-#18508 ordering mechanism.