Bazel: allow installer script to install into multiple directories, in parallel

[redsun82]

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[redsun82]

I might have overengineered it, but I think we could shave off several seconds in the final install steps for full dists if we employ this.

[criemen]

I might have overengineered it, but I think we could shave off several seconds in the final install steps for full dists if we employ this.

You think so? I'd have guessed that ripunzip already employs parallelity, so there's not so much to be gained by calling it multiple times in parallel. I guess we can see if copying the extractor files could be sped up like this. I'll put this onto my review queue, but at lower priority. I guess this'd need somewhat significant changes to how we build pack groups to be used for single-target intree dists like target/intree/codeql.

[redsun82]

It might be that most of the gain would rather come from avoiding the multiple bazel run overhead, rather than using parallelism. We can always test that out and remove the code for parallel runs if it doesn't really matter. In any case, yes, this can definitely wait.

[criemen]

Ah, a single intree-dist is currently already modelled by a single bazel run, it's just split dists that invoke bazel run multiple times (once for nolang, once for every extractor requested)