codeintel: Use git.ListDirectoryChildren
Created by: ryanslade
This change is needed so that we no longer send raw git commands to gitserver form outside of the vcs/git package.
We ended up just copying the implementation from lib/codeintel/pathexistnce as the vcs/git package is not visible to that package and we don't want out internal vcs/git package to rely on packages in codeintel/lib.
Part of https://github.com/sourcegraph/sourcegraph/issues/27909