MaterialX search paths seem to expect different locations for finding nodes and loading implementations

[laserallan]

Description of Issue

When I build the MaterialX plugin and set PXR_MATERIALX_STDLIB_DIR in my build I get errors.
If I point it to the install_dir the nodes are loaded properly but the code generation fails since it can't find the implementations.
If I point it to install_dir/libraries, the nodes are not found

Note install_dir refers to your install_dir passed to build_usd.py

Steps to Reproduce

1. Build with PXR_MATERIALX_STDLIB_DIR to the build directory
2. Make sure any other environment variable helping MaterialX finding nodes and implementations are unset (I never managed to get any of them to work either)
3. Reference a simple MaterialX material into a scene and apply it to an object

Now you will get errors like this:
ERROR: Usdview encountered an error while rendering.Failed to get source code from file 'lights\genglsl\mx_point_light.glsl' used by implementation 'IM_point_light_genglsl'

My workaround which may or may not be great is setting PXR_MATERIALX_STDLIB_DIR to install_dir and adding searchPath.append(mx::FilePath(PXR_MATERIALX_STDLIB_DIR)/"libraries"); to the function HdSt_ApplyMaterialXFilter where the search paths are created.

I've attached a file reproducing the issue. Make sure you update the path to standard_surface_marble_solid.mtlx in the MaterialX package.
materialx_issue.zip

System Information (OS, Hardware)

Windows 10, NVIDIA GPU, Visual Studio 2017, AMD 64

Package Versions

21.05 RC 1

Build Flags

MaterialX enabled, PXR_MATERIALX_STDLIB_DIR set to the install_dir

[jilliene]

Filed as internal issue #USD-6653