`git lb-checkout` should allow only check out of _packages_

Although git allows checkout of individual files or any subdirectory, for git lb-checkout we only care about whole packages.

The list of packages can be obtained with

git ls-tree --name-only -r <commit-ish> | sed -n 's@/CMakeLists.txt@@p'

which could be wrapped in a git lb-checkout --list <commit-ish>.