loader: Do not remove duplicated layers

loader: Do not remove duplicated layers

Linux can support multiple ABIs, but Vulkan Layer manifest does not
allow to specify different library paths, based on the ABI.

As a solution, for ICDs, we can simply create multiple manifests, one
per ABI, and the Loader will try them one by one until it finds the
library that is compatible with the executable class.

Instead, for Vulkan Layers, this method doesn't work because the Loader
will discard the manifests that have a duplicated layer name.

To add support for multiple ABIs to Vulkan Layers, and to make the
behavior similar to the ICDs, with this commit we remove the duplicated
layer name check.
Instead we add to the output list all the Vulkan Layers that we find,
and only when we are actually going to dlopen them, we discard the
layers with the same name that we already successfully opened.

Fixes: #155

Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>