Factor out rule targets into a separate module

[snowleopard]

A refactoring in preparation of #5025.

• Create a new module Targets for representing rule targets. For now, it only represents file targets but the interface is designed to extend the module to support directory targets too. For example, various function arguments are called ~file instead of more usual ~f because directory targets will require a new argument ~dir.
• Rename Dune_rules.Targets to Dune_rules.Targets_spec to avoid confusion.

The only slight change of behaviour is around pretty printing of targets in an error message in rule.ml: it used to simply turn paths into a string but the new Targets.pp uses Dpath.describe_target as in other places where we pretty print targets.