Optimize GPKG and SQLite sublayer finding and filtering

[gacarrillor]

For instance,

• Applying alphanumeric filter before anything else, which can discard layers soon, without applying other filters.
• Using QgsProviderRegistry.instance().providerMetadata('ogr') to get lists of tables filtered, for instance: conn.tables(flags=QgsAbstractDatabaseProviderConnection.Aspatial).