directory_layout: remove outdated checks for old DAG hash

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.

0 files changed, 0 insertions, 0 deletions