This removes all but one usage of runtime hash. The runtime hash was being used to write
historical lockfiles for tests, but we don't need it for that; we can just save those
lockfiles.
- [x] add legacy lockfiles for v1, v2, v3
- [x] fix bugs with v1 lockfile tests (the dummy lockfile we were writing was not actually
a v1 lockfile because it used the new spec file format).
- [x] remove all but one runtime_hash usage -- that one needs a small rework of the
concretizer to really fix, as it's about separate concretization of build
dependencies.
- [x] Document the history of the lockfile format in `environment/__init__.py`
7c1d566959