Diverging opret/tapret assignments are skipped for an outpoint

[dr-orlovsky]

The existing API which allows to list contracts and specific assignments to a given bitcoin tx outpoint works only withing a scope of given single-use seal method, since it uses XOutputSeal (which includes closing method information) instead of XOutpoint (which doesn't). Thus, when you have diverging tapret/opret assignments to the same outpoint, one type of them is always ignored, which leads to a loss of assets.

Test demonstrating the issue: RGB-WG/rgb-tests#24 (NoInput variant of the failure is caused by this problem)