Rely on libsdl2-dev (>= 2.0.4) providing a CMake "config-file package"

This means we no longer need to copy/paste a FindSDL2 module, removing
some redundant code; it also bypasses a regression in libsdl2-dev
resulting from the introduction of multiarch support, which broke
FindSDL2's assumptions, causing FTBFS.

Mitigates: #951087