CPS provides "default targets" as a user convenience which can be linked
by specifying only the package name. CMake implements this via a utility
`INTERFACE` target. However, because it is not namespaced, this is not a
valid requirement, which prevents exporting anything linking to this
target to CPS.
Work around this by introducing a new (internal) target property which
replaces a target with its link interface targets at export time.
