Fixed the strategy how to approach Npgsql.EntityFrameworkLegacy.

If you request DbProviderServices, try to load tightly coupled Npgsql.EntityFrameworkLegacy first.

And then query NpgsqlServices.TargetProviderServices property.

We can know supported version of DbProviderServices by checking TargetProviderServices.

We provide NpgsqlServices.Instance if we share same version of EntityFramework library.