add opam loader

Summary:
added an opam loader

Related to T3425

Test Plan: will add tests later using a local opam repository as it's done in the lister

Reviewers: Reviewers, vlorentz, ardumont

Reviewed By: Reviewers, ardumont

Subscribers: ardumont, vlorentz

Maniphest Tasks: T3425

Differential Revision: https://forge.softwareheritage.org/D5975