Add package folding for build graph

[manuuurino]

First, I'd like to thank you for this awesome project, the build graph visualization looks unique.

I'm using a flake with impermanence module. When building, the impermanence module creates separate packages for each persistent path.
The build graph becomes cluttered with these path packages, which can appear to idle for extended periods (up to ~2 minutes in my case), making it confusing whether the build is stuck.

Could we add the ability to fold certain packages in the build graph visualization? Specifically, I suggest adding a parameter based on a regex pattern. This would allow users to collapse packages that clutter the visualization.

One approach could be adding a flag like --fold-pattern="<regex>".
This could show a "(N x)" after the dependency to indicate how many packages are folded.

Would this be something you'd consider implementing? Perhaps one day if I feel comfortable with functional programming I could try a PR.

[maralorn]

I guess adding a config option to not show any leaves below a certain element is possible.
I think for the time being there are other improvements that seem more urgent to me, but I am not strictly opposed.