This is a major rework of Spack's core core `spec.yaml` metadata format. It moves from `spec.yaml` to `spec.json` for speed, and it changes the format in several ways. Specifically:
1. The spec format now has a `_meta` section with a version (now set to version `2`). This will simplify major changes like this one in the future.
2. The node list in spec dictionaries is no longer keyed by name. Instead, it is a list of records with no required key. The name, hash, etc. are fields in the dictionary records like any other.
3. Dependencies can be keyed by any hash (`hash`, `full_hash`, `build_hash`).
4. `build_spec` provenance from #20262 is included in the spec format. This means that, for spliced specs, we preserve the *full* provenance of how to build, and we can reproduce a spliced spec from the original builds that produced it.
**NOTE**: Because we have switched the spec format, this PR changes Spack's hashing algorithm. This means that after this commit, Spack will think a lot of things need rebuilds.
There are two major benefits this PR provides:
* The switch to JSON format speeds up Spack significantly, as Python's builtin JSON implementation is orders of magnitude faster than YAML.
* The new Spec format will soon allow us to represent DAGs with potentially multiple versions of the same dependency -- e.g., for build dependencies or for compilers-as-dependencies. This PR lays the necessary groundwork for those features.
The old `spec.yaml` format continues to be supported, but is now considered a legacy format, and Spack will opportunistically convert these to the new `spec.json` format.
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