loader: Remove duplicated delimiting chars in paths
loader: Remove duplicated delimiting chars in paths

Before loading a manifest file, its full path is strcmp() with already
loaded ones in order not to load the same file twice. While being simple
and efficent, this mechanism is not able to detect subtle duplicates
like these:

/usr/share/vulkan/icd.d/intel_icd.x86_64.json
/usr/share//vulkan/icd.d/intel_icd.x86_64.json

This patch ensure that searched paths do not contains such duplicated
directory delimiting characters in order to avoid this kind of problem.

Fixes #2331
Fixes #2629

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers