Nix extract function should return packageFile if lock file exists

[rarkins]

Describe the proposed change(s).

Currently Renovate discards any package file without detected dependencies, however we should return it (with empty dependencies) also if a lock file exists.

Discussed in #29263

Reproduction (of it working with some local modifications): https://github.com/renovate-reproductions/29263/pull/1/files

[RahulGautamSingh]

We will need to add and export extractAllPackageFiles function in the nix manager to handle asynchronous operations required for this fix, right?

[rarkins]

I don't think that's necessary

[RahulGautamSingh]

We can't add file reading to the extractPackageFile fn, since it is defined as normal function. Whar other approach would be to check the lockfile exists?

[rarkins]

Maybe you're right. But I'll self assign this as I solved it earlier and have a stash saved somewhere

[viceice]

We can't add file reading to the extractPackageFile fn, since it is defined as normal function. Whar other approach would be to check the lockfile exists?

extractPackageFile can be async too.