Add support for validating packets

[richfitz]

This PR adds support for validating that packet contents are ok in the archive. This is about the 5th time I've tried to implement this and the first one that I have liked, made possible by our current use of cli, and the pattern of converting dots into a query.

I've held off from implementing repair here, as there are a few options for doing that (pulling from the filestore is trivial, finding other local files is easy enough but will miss many, pulling from a location should be the fallback).

I'm going to work up a second pull request that adds high-level docs and a workflow that encourages users to try running this.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (9a1c9ec) 100.00% compared to head (cc5d450) 100.00%.

❗ Current head cc5d450 differs from pull request most recent head 9311ea0. Consider uploading reports for the commit 9311ea0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #92   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           37        38    +1     
  Lines         3089      3147   +58     
=========================================
+ Hits          3089      3147   +58     

Files Changed	Coverage Δ
R/outpack_hash.R	100.00% <ø> (ø)
R/location.R	100.00% <100.00%> (ø)
R/outpack_index.R	100.00% <100.00%> (ø)
R/validate.R	100.00% <100.00%> (ø)

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