Do not resolve path in abs_path.
We used to do that with realpath in `track', but it wasn't really the intention.
It could make perfect sense if a user wanted to track a symlink in $HOME
that links to somewhere else in the system.
By resolving the symlinks we disallowed that.
No need to use the `err` function, abs_path is internal
Simplify abs_path
Simplify abs_path even further
add symlink directory check impl and test