Lock for local dependency uses absolute path

[shunueda]

Related: #13877

Expected Behavior

in dune-project:

(pin
 (url ../foo)
 (package (name foo)))

locking (dune pkg lock) this should produce:

(source
 (fetch
  (url ../foo)))

Actual Behavior

Instead it uses an absolute path:

(source
 (fetch
  (url
   file:///Users/...<absolute path>.../foo)))

This makes the lock not portable.