[TML-1610] adjust behavior for default namespace to avoid differences during migrate diff (#5699)
This PR addresses https://github.com/prisma/prisma/issues/28240.
When creating the db schema for a schema file we do NOT include the
`CREATE SCHEMA public ...` statement. But when creating it from the
migrations (aka shadow db) it is included.
Due to this when creating a diff between the two as in migrate diff an
actual diff in the db schema is observed.
This bug only shows up when using `migrate diff` with the `--exit-only`
or `--script` options because of a missing implementation for schemas in
the drift summary.
This PR fixes the issue by changing the way how we determine the
`explicit_namespace` for rendering SQL statements (see
https://github.com/prisma/prisma-engines/pull/5614) and ensuring that
the default namespace is always part of the namespace list - no matter
how the db schema is derived.