[red-knot] Use relative paths in diagnostics rather than absolute paths

[AlexWaygood]

Diagnostics are currently reported with absolute paths, which makes them quite verbose even if you specify --output-format=concise. E.g. here's a diagnostic I got running red-knot from my /Users/alexw/dev/typeshed-stats directory:

error[lint:invalid-parameter-default] /Users/alexw/dev/typeshed-stats/scripts/runtests.py:17:5: Default value of type `EllipsisType` is not assignable to annotated parameter type `Literal[False]`

It would be nicer if we used the relative path instead, which is what Ruff does:

error[lint:invalid-parameter-default] scripts/runtests.py:17:5: Default value of type `EllipsisType` is not assignable to annotated parameter type `Literal[False]`

[MichaReiser]

Yes! @BurntSushi this one would be great to get into the alpha.

One question is whether the paths should be relative to the project root or the current directory. I think it should be the current directory, so that the paths are "clickable"