Decouple GLSL and MSL "libraries" location.

[ld-kerley]

Currently there is a dependency between the GLSL and MSL shader generation modules, because both targets share source code (which is great), the MSL shader generator uses files from the genglsl libraries locations.

This becomes a small problem when building with USD, if the MaterialXGenGLSL module is not built, as the code uses MaterialXGenGLSL::TARGET to generate the genglsl component in the file path.

This PR aims to address this, by more correctly decoupling the MSL and GLSL shader generators, while still allowing the source code, and there only be a single copy of the code in the source repo, as well as make some of the cmake dependency tracking more robust.

All files shared between genglsl and genmsl have been moved from the genglsl location to a shared _hwCommon location.

The libraries CMakeLists.txt has been updated to "build" a staged folder from the source folder, where

• Only the shader generators that are enabled will have their source code staged to the libaries folder. This will result in smaller installations, where only one shader generator is desired.
• The _hwCommon files are correctly copied to both genmsl and genglsl.
• The OSL legacy closures file switch is also staged in this build location, this unifies this logic in one place.

This also slightly simplifies the testing logic as we don't have to replicate the OSL legacy closure logic, and instead can just copy the staged files to the location for testing. The dependency tracking for copying the testing files is also fixed, so iterative edits to any of the files in libraries should be automatically installed to the testing location, previously this location would need to be deleted for cmake to copy the files.