The overlay infrastructure now supports declaring that the root
directory of some builds is a subfolder of a given repository.  This
allows overlay-writers to override the repository and branch (and
override the subfolder with "") to indicate that the repository should
be cloned from elsewhere.

This should hopefully provide a relatively seamless experience for
overlay writers who need to update submodules of projects like
fiat-crypto.  In service of that, we split off
ci-{coqutil,kami,riscv_coq,rupicola}

N.B. We avoid duplicating downloads of parent projects when possible.

N.B. Subprojects should be under their immediate parent, otherwise
overlays are applied incorrectly.

N.B. We use -PARENT directories on CI

This way sibling jobs don't mess with each others' timestamps.

We also provide `USE_CI_DIRECTORY_STRUCTURE` to allow those downloading
artifacts to reuse the downloaded directories.