Our vLLM plugin checks out sources of CUTLASS, FlashMLA, and vLLM's flash-attention. The sources are currently placed in an ephemeral directory GIT_REPOS_DIR. The content vanishes when the builder container is stopped. The should be vendored in the sdist so we have a self-contained source distribution. It makes it much easier to debug and reason about a build, too.

xformers and Torch use a similar approach.

We should remove GIT_REPOS_DIR and vendor all checkouts into their sdists.