Currently, specs on buildcache mirrors must be referenced by their full description. This PR allows buildcache specs to be referenced by their hashes, rather than their full description.
### How it works
Hash resolution has been moved from `SpecParser` into `Spec`, and now includes the ability to execute a `BinaryCacheQuery` after checking the local store, but before concluding that the hash doesn't exist.
### Side-effects of Proposed Changes
Failures will take longer when nonexistent hashes are parsed, as mirrors will now be scanned.
### Other Changes
- `BinaryCacheIndex.update` has been modified to fail appropriately only when mirrors have been configured.
- Tests of hash failures have been updated to use `mutable_empty_config` so they don't needlessly search mirrors.
- Documentation has been clarified for `BinaryCacheQuery`, and more documentation has been added to the hash resolution functions added to `Spec`.
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