Fix sdl2 dependents

[cbezault]

Resolves #6152.

[grdowns]

Perhaps it might be best to write this into the vcpkg_cmake_wrapper.cmake to populate SDL_LIBRARY from SDL_LIBRARY_RELEASE and SDL_LIBRARY_DEBUG so that less ports have to be touched by this? It might also aid authoring ports in the future.

EDIT: Nevermind, this pattern is quite prevalent.

[Neumann-A]

The patch should use find_package instead of find_library.
SDL2 exports targets and linking against those is probably desirable.

[cbezault]

@Neumann-A, I agree, thanks for taking a look.

[cbezault]

Same as #5948