keep checkpoints file aside (c.FileCheckpoints)

[gsemet]

Follow up of jupyterlab/jupyterlab#5809.

We are kind of getting polluted by all these checkpoints files, and setting them aside with

c.FileCheckpoints.checkpoints_dir =...

helps but the filename is only based of the opened file basename, not the path. So if two files are opened, with the same basename, they will overwrite the same checkpoint files.
Why not simply add the path in the checkpoint filename (or base64 encode it)

[jasongrout]

I can see this happening a lot particularly with the very common UntitledX.ipynb files.