Add (partial) file set export to CPS. For now, only headers ("includes")
are supported, as that is the only overlap with the specification.
(Source file sets are not supported by CPS at this time, and the hope is
that C++ modules will never be supported, as that information should
already be available via the `cpp_module_metadata` information.) CPS
also supports "embeds", but CMake doesn't (yet).
This also introduces our first official use of CPS extensions, used to
record the CMake file set name.
Modify install exports to make use of the install generator's captured
diagnostic context. Modify build exports to capture and use a diagnostic
context. This allows diagnostics issued by export file generation to use
the diagnostic context from the instigating command, and to provide a
backtrace to the same.
CPS package metadata involves a fair amount of passing around a
particular set of values, as well as shared argument handling, which is
only going to increase as additional features are added. In order to
reduce code duplication (now and going forward), create a helper class
to handle the shared argument parsing and to serve as a container to
pass metadata values.
Matthew Woehlke
Vito Gamberini
Kitware Robot