Default is shared linkage, also provide -static and -headers-only (bring
your own SDL) targets.

Also ditch the FindModules hanging out in the CMake folder. SDL ships
with appropriate config mode module discovery and CMake ships with
FindModules for SDL components.

This allows the "headers-only" target to be installed even if SDL wasn't
found during configuration, enabling a "bring your own SDL" style of
development for devs who vendor specific SDL versions or binaries in
their codebase.

* Update Ubuntu hosts before installing packages
* Convert env paths to CMake native
* Drop SDL versions which don't ship configs on Windows
* Pass locally downloaded SDL packages to find_package(PATHS)