check-utils: add hasBuilt

[blaggacao]

This implementation of hasBUilt depends on a very simple IFD (builtins.pathExists drv). It works well for successful builds, but currently build-errors for failing ones, despite of the use of builtins.tryEval.

I'm investigating if this behaviour of builtins.tryEval can be considered an upstream bug, as long as IFD, per se, are not forbidden.

(opinion milage may vary)

[gytis-ivaskevicius]

Can we turn this into an actual PR and see what @zimbatm thinks? (Don't forget to add success/failing cases to the example and paste resulting output)

[blaggacao]

I'll try to have some conversations about tryEval catching or nor catching build errors, first. This is going to be a long hauler. In the current state of tryEval it does not work and upstream opinions seem divided.

[Mic92]

Is check-utils part of the public api of this flake? @zimbatm

[gytis-ivaskevicius]

Yes

[Mic92]

It's not part of the documentation though: https://github.com/numtide/flake-utils

[zimbatm]

It was added by blaggacao and gytis. I haven't used it so far.

[zimbatm]

I think this effort has moved elsewhere