This removes the sfml- prefixed targets from the export set. The sfml-
prefixed targets are still available within the build tree but not to
downstream users thus making this an API breaking change when compared
to the 2.x releases. To keep things consistent, usage of the sfml-
targets were replaced with their namespaced counterparts.
This has a number of benefits:
1. It's more idiomatic. Modern CMake libraries are expected to
have namespaced targets.
2. Namespaced targets are less likely to collide with user-defined
targets. No one will accidentally define a SFML:: target.
3. If a namespaced target is not found by CMake, configuration
will immediately stop.
