deprecate pkg path in flagd module

[bluebrown]

Hi, I am experimenting with this library and open feature sdk itself.
I feel like the import path is a bit awkward to use. I can see the pkg
path has been deprecated in the SDK already, but in this package not.

I have to import it like this. Which isn't nice, imo. And its also fiddley
to find out. You have to go and read the files and packages named to
understand that there is some naming inception going on.

import (
	flagd "github.com/open-feature/go-sdk-contrib/providers/flagd/pkg"
)

Perhaps this should just be github.com/open-feature/go-sdk-contrib/providers/flagd.
And then use internal directories to define the public API boundaries,
instead of pkg.

[toddbaert]

Ya I agree. We don't even need to support the old one, since the provider is not 1.0+ and it's not likely to be imported in too many places; we can just make this a breaking change instead of deprecating the old like we did in the SDK.