Multiple pin-depends in opam files on a single local package fails

[ElectreAAS]

Expected Behavior

Having multiple opam-defined packages in a repo who all have a pin-dependency on a local package should unify all these (probably equal) constraints and build correctly

Actual Behavior

Error: local package <pkg> cannot be pinned
See reproduction PR for exact behaviour

Reproduction

• PR with a reproducing test: Reproduction test for multiple-pin-depends bug #12991

[ElectreAAS]

Found while trying to build irmin on their eio branch