We previously had checks in `directory_layout` to check for build-dependency
conflicts when we weren't storing build dependencies. We don't need
those anymore; we can just rely on the DAG hash now that it includes everything
we know about each spec.
- [x] Remove vestigial code for checking installed spec against concrete spec
in `ensure_installed()`
- [x] Remove `SpecHashCollisionError` -- if specs have the same hash now, they're
the same as far as `DirectoryLayout` should be concerned.
- [x] Convert spec comparison to `dag_hash()` comparison when adding extensions.
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