Make googletest, SPIRV-Headers, SPIRV-Tools submodules

I noticed that building requires me to clone googletest manually. Is there a reason why it isn't a git submodule?

Inspect the repository's dependency and build configuration to see how googletest, SPIRV-Headers, and SPIRV-Tools are currently acquired. Check existing submodule conventions before deciding how each dependency should be declared. Done means a fresh clone can initialize the required dependencies through the repository's normal setup process without manual cloning.

Written by the indexing model from the issue text.