Source extensions fail to compile when nested

[christopherswenson]

Source extensions are not applied during compilation for nested views.

E.g.

run: duckdb.table('malloytest.state_facts') -> {
  nest: foo is {
    extend: {
      dimension: y is 1
    }
    group_by: y
  }
}

This is true whether or not the nested view is defined inline or referenced.
This also applies to join_xxx directly in a query.