Create a 'find' like utility for browsing available attribute paths

[jwiegley]

This would be a custom normalization routine (sensitive to loops, like forceEffects) that would recursively descend a top-level tree (such as <nixpkgs>) in order to report every attribute reachable from the top-level set. If we accept arguments in the spirit of GNU find, we could search for all paths under haskellPackages, for example.

Once this exists and is implemented as a "recursing finder" (likely using my hierarchy library), then we could implement a test that recurses through the entire tree concurrently, and evaluates every attribute value and compares it to the evaluation of the same attribute using nix-instantiate.

[jwiegley]

If @mightybyte uses streaming to implement fetchTarball, then I'll likely port hierarchy to only need streaming as well.

[mightybyte]

@imalsogreg is working on fetchTarball. I probably won't get around to doing anything with the streaming port any time soon.

[jwiegley]

@mightybyte No rush. I finished my port of hierarchy to streaming, and will release it soon to Hackage as streaming-hierarchy. I intend to use it in our implementation of filterSources too.

[jwiegley]

I've made progress on this issue, available as a new --find option. However, I haven't figured out an intelligent way to know when to recurse sub-attrsets. Clearly stdenv shouldn't be recursed every time it's encountered. I have some code to try to attempt to force everything breadth-first, so that we never descend into a thunk we've already forced before. However, this causes haskellPackages to appear under bootPkgs, which seems confusing to me.

I'm beginning to wonder if we'll need a manual map to help guide the report so that it makes the most sense. Or, if we should expect it just to be used on subtrees most of the time, and so excessive reporting at the top level is just "as designed".