Some distros ship MangoHub vulkan layer json with bare shared object name
instead of an absolute path. This breaks environment config as
`MinecraftInstance::createLaunchEnvironment()` seems to require absolute
path of `libMangoHud.so`.
Since we already have `MangoHud::findLibrary()` lying around, use that to
figure out where `libMangoHud.so` really is. In case of a platform that
doesn't support `dlopen()`, fallback to old behavior and return the path
verbatim as it is recorded in vk layer json.
Signed-off-by: Tianhao Chai <cth451@gmail.com>
