* Limit `derived_view_schemas` SQL generator to actual view directories.
* Fix the `derived_view_schemas` SQL generator to get the view schemas by dry-running their latest SQL and/or from their latest `schema.yaml` file. Getting the schema from the currently deployed view wasn't appropriate because it wouldn't reflect the latest view code.
* Rename `View.view_schema` to `View.schema`.
* Change `View` so its schema dry-runs use the cloud function (CI doesn't have permission to run dry-run queries directly).
* Apply partition column filters in view dry-run queries when possible for speed/efficiency.
* Don't allow missing fields to prevent view schema enrichment.
* Only copy column descriptions during view schema enrichment.
* Only try enriching view schemas from their reference table `schema.yaml` files if those files actually exist.
* Change `main_1pct` view to select directly from the `main_remainder_1pct_v1` table, so the `derived_view_schemas` SQL generator can detect the partition column to use and successfully dry-run the view to determine its schema.
* Formalize the order `bqetl generate all` runs the SQL generators in.
* Have `bqetl generate all` run `derived_view_schemas` last, in case other SQL generators create derived views.
* Fix `Schema._traverse()` to only recurse if both fields are records.
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