atdml: provide option to write output files in the same folder as the source

Problem: the following dune file works but produces error messages referencing ./types.atd rather than the path from the workspace root or project root:

(rule
  (targets types.ml types.mli)
  (deps types.atd)
  (action
    (run atdml %{deps})
  )
)

A solution is to change directory using (chdir ...) which takes care of adjusting the %{deps} paths:

(rule
  (targets types.ml types.mli)
  (deps types.atd)

  ; Run this preprocessing command from the project root so as to get
  ; usable paths to the source file in error messages.
  (action
    (chdir %{workspace_root} (run atdml %{deps}))
  )
)

but now the problem is that the targets are created at the workspace root (<root>/types.ml) instead of at <root>/path/to/types.ml which is not where dune expects them.

Proposed solution:

• Add an option to atdml that makes it write the output files in the same folder as the atd source file (I like this better than making it a default, even if backward compatibility wasn't an issue)

Start by locating atdml's command-line option handling and output-file generation, then reproduce the issue with the shown dune rule and a nested types.atd source. Done means an option causes generated files to be written beside the source while diagnostics still use project-root paths and dune finds the declared targets.