Previously a fileset with `$<$<CONFIG:Debug>:some_file>` would show up
as-is (with escaping) in the build directory export. Instead, evaluate
all fileset entries as generator expressions and list them as they are
similar to the installation information.
There is no reason to allow these properties to be manipulated by user
code. Instead, use the stored visibility on the fileset objects to
derive what these properties should contain.
