Make the backend overridable service discovery also check the database type for an overridden service

Problem/Motivation

For database drivers that override an other database driver, such as https://www.drupal.org/project/pgsql_fallback → , have to create their own implementation for backend overridden services. The PostgreSQL fallback driver has to create its own service for the pgsql.views.date_sql service. This is problematic because the PostgreSQL fallback driver lives in a module and it is not enabled by default and therefor the backend overridden service does not exist until it is enabled.
Core now only uses the driver name to check for a backend overridden service. The PostgreSQL fallback driver has the driver name "PgsqlFallback", not "pgsql".

Proposed resolution

Make the backend overridable service discovery also check with for the Connection->databaseType() service override when there is no override for Connection->driver() service.

Remaining tasks

TBD

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

TBD