Add modules to the sdl dependency class, e.g.: image, mixer, net, etc.

Currently, you have to use strangely-named pkg-config files instead of modules like with Boost, Qt, etc. This is a cause of frequent confusion on IRC and on issues such as https://github.com/mesonbuild/meson/issues/2245.

Locate Meson's SDL dependency class and compare its module handling with the existing Boost and Qt dependency implementations. Done means SDL modules such as image, mixer, and net can be requested directly instead of through confusingly named pkg-config files.

Written by the indexing model from the issue text.