[vulkan] Add more robust search paths to Vulkan loader library

[derek-gerstmann]

Also, add HL_VK_LOADER_LIB env var to override search paths.
Add docs for HL_VK_LOADER_LIB env var.

This PR makes it easier to locate the Vulkan loader library when running inside an emulated environment, or in a sandbox, or to test a developmental version of the Vulkan loader lib.

[alexreinking]

I have a couple library-ordering nits

[derek-gerstmann]

Thanks! Yes, I agree ... I'll change the ordering for dynamic linking to match the docs for the Vulkan Application Interface:
https://vulkan.lunarg.com/doc/sdk/1.4.321.1/windows/LoaderApplicationInterface.html

Windows:

• vulkan-1.dll (first choice)

Linux:

• libvulkan.so.1 (first choice)

OSX:

• libvulkan.1.dylib (first choice)

Windows Dynamic Library Usage
On Windows, the loader library encodes the ABI version in its name such that multiple ABI incompatible versions of the loader can peacefully coexist on a given system. The Vulkan loader library file name is vulkan-<ABI version>.dll. For example, for Vulkan version 1.X on Windows the library filename is vulkan-1.dll. This library file can typically be found in the windows\system32 directory (on 64-bit Windows installs, the 32-bit version of the loader with the same name can be found in the windows\sysWOW64 directory).

Linux Dynamic Library Usage
For Linux, shared libraries are versioned based on a suffix. Thus, the ABI number is not encoded in the base of the library filename as on Windows.

On Linux, applications that have a hard dependency on Vulkan should request linking to the unversioned name libvulkan.so in their build system. For example by importing the CMake target Vulkan::Vulkan or by using the output of pkg-config --cflags --libs vulkan as compiler flags. As usual for Linux libraries, the compiler and linker will resolve this to a dependency on the correct versioned SONAME, currently libvulkan.so.1. Linux applications that load Vulkan-Loader dynamically at runtime do not benefit from this mechanism, and should instead make sure to pass the versioned name such as libvulkan.so.1 to dlopen(), to ensure that they load a compatible version.

MacOs Dynamic Library Usage
MacOs linking is similar to Linux, with the exception being that the standard dynamic library is named libvulkan.dylib and the ABI versioned library is currently named libvulkan.1.dylib.