EMSUSD-622 - As a user, I'd like to add a relative sublayer on a anonymous layer

[vlasovi]

No description provided.

[pierrebai-adsk]

I know that it is not multi-thread safe anyway, but just for order-of-initilization of C++ globals, it is a better practice to put the static variable inside a static function that returns it because the C++ language standard guarantee hat the static will be initialized on first call (which avoid the order of init of globals problem) and ensure that this init is thread-safe.

So something like:

using RelativePaths = std::map<PXR_NS::SdfLayerHandle, std::set<ghc::filesystem::path>>;
static RelativePaths& getRelativePaths()
{
    static RelativePaths sPaths;
    return sPaths;
}

[pierrebai-adsk]

Small change requested to make it safer vs C++ link order.

Otherwise, looks good to me.