Split out content loading from XML emitter. #3936.
Introduce separate `Filters` module. #3940.

The motivation is to re-use the functionality in
`Filters::CompositeFilter` (nee `Transformers::Filters`) between
`EnumTransformer` and `Contents`. Since we will eventually be adding
filters which could work in either context, moving the existing
filters to a new namespace makes sense also.

This commit also adds the new `Filters` module to the search path for
unknown constants when reading configuration files. This should ensure
that moving the filters has no impact on deployed configurations,
since they all use the unqualified name.
Implement filters option for `Contents`. #3940.
Add named filter set concept. #3640.

These are primarily intended to be used with the XML emitter at this
point.
Add filter option to emit-xml. Closes #3640.

This includes a default setting in the configuration, and a mechanism
for defeating that default setting.
Log changes for #3940.