Appendices should be optional

[mwoehlke]

Right now, the specification does not provide much guidance whether appendices should be loaded.

The behavior of CMake 4.0 is to try to load appendices that provide requested components, or to try to load all appendices when not given specific component requests.

It might be useful for packages to have control over this latter behavior, e.g. an optional attribute that specifies that an appendix should be ignored unless it provides a requested component.

[mwoehlke]

the specification does not provide much guidance whether appendices should be loaded

This is a bit overly pessimistic. In fact, we do say that "tools are permitted and encouraged to ignore the information in any such supplemental CPS files as they may determine is not relevant to the user’s needs."

Nevertheless, packages might find it useful to be able to give guidance whether an appendix should be loaded or not in the absence of consumer guidance.