USD CMake files included into the manifest

[boberfly]

Before I forget again, could we bring in the USD cmake stuff into the manifest? I found that I needed it to build USD-related stuff against the Gaffer releases and it needed these things.

I believe it is:
pxrConfig.cmake
cmake/pxrTargets.cmake
cmake/pxrTargets-release.cmake

It looks a bit messy in the root directory if that's a concern, I wonder if we could massage them into the lib/ directory like the other cmake stuff and run a sed over the pathing...

[johnhaddon]

Sounds reasonable to me, but I'd really like to avoid cluttering up the top-level directory namespace. I'd also be concerned that some .cmake files might be baking full paths in (I saw that with MaterialX), so wouldn't be relocatable as is needed when people install Gaffer. Happy to accept a PR if those two specific issues can be addressed.