The views in pg_catalog and information_schema provide introspection
over the logical schema (pure SQL view) however when testing/debugging
it is also important to see the descriptor IDs and also other fields
of descriptors.

This patch provides help for debugging with the following new virtual
tables:

- `crdb_internal.table_columns`: detailed column IDs and properties
- `crdb_internal.table_indexes`: detailed index IDs and index properties
- `crdb_internal.index_columns`: what columns are indexed and how
- `crdb_internal.backward_dependencies`: which other descriptors each descriptor depends on
- `crdb_internal.forward_dependencies`: which other descriptors each descriptor is depended on by

These virtual tables are intended to display the contents of
descriptors without any data transformation, so as to reveal any error
in the descriptors, if any.

Debugging/fsck tools can later use these to check schema validity.

... because it was incomplete (missing FK relationships).

The two new tables `crdb_internal.forward_dependencies` and
`crdb_internal.backward_dependencies` can be used instead.

Prior to this patch, view dependency analysis during CREATE VIEW was
broken because it would only check dependencies after plan
optimization, i.e. possibly after some dependency were lost. For
example, with the queries:

```sql
CREATE VIEW v AS SELECT k FROM (SELECT k,v FROM kv) -- loses dependency on kv.v
CREATE VIEW v AS SELECT k,v FROM kv WHERE FALSE -- loses dependency on kv
```

This patch addresses the issue as follows:

- the dependencies are now collected during the initial construction
  of the query plan, before any optimization are applied. This way we
  ensure the dependency tracking is complete.
- a migration is implemented that will fix any view descriptor and
  corresponding dependency information that was populated prior to
  this fix.