Packages: Add integration between FetchContent and find_package()

See the associated issue (#21687 (closed)) for detailed discussion of the motivation and scope of this change. The short version is to allow find_package() and FetchContent_MakeAvailable() work better together. In particular, the latter can call the former to satisfy a dependency, or the former could be redirected to the latter to avoid pulling in an external dependency that FetchContent has already provided.