Improve correctness of sourceforge-ls

Summary:
Before this patch, these useful tools did not ignore duplicated
projects in /projects/ the namespace, did not account for
non-/p/ namespaces nor suprojects, as rare as they might both be.

Reviewers: Reviewers, ardumont

Reviewed By: Reviewers, ardumont

Maniphest Tasks: T735

Differential Revision: https://forge.softwareheritage.org/D5294