fix Can't open a directory named checkpoints #12408

[eastonsuo]

#https://github.com/jupyterlab/jupyterlab/issues/12408

Resolves: #950

[Zsailer]

Hey @eastonsuo, apologies that the team hasn't responded here.

This has been a long standing issue in Jupyter-Server that requires a bit more nuanced work to allow these paths. Unfortunately, as this PR stands, it solves the immediate issue, but removes critical existing APIs. To fully address the issue, we'd need to think carefully about deprecating the old APIs and special-casing the checkpoints+trust endpoints.