Use relative paths for checkouts symlinks

[gordonsyme]

I run a lot of code in docker containers, in a VM on my laptop while using the host machine for editing. Using relative paths for checkouts allows the symlinks to remain valid when mounted into other filesystems.

I've assumed that modules must always be child directories of the parent project, if that's not the case I also have a version of this that uses relative paths when that's true and absolute paths otherwise.