runtime: Do not dlopen layers if it is not needed
Currently the Vulkan layer loader does not support having multiple layer manifests with the same "name" field. This can be a problem when you want to support multiple ABIs.
For this reason some layers, like vkBasalt, use a "library_path" with just the filename of the library. In this way, at execution time, the correct library will be chosen automatically.
To keep this behavior also inside the container, we avoid using the absolute path if we have just the filename.
Partial workaround for #39 (closed)
/cc @smcv
Edited by Ludovico de Nittis