Move all files from pkg/openfeature to openfeature.

Move the package from being
github.com/open-feature/go-sdk/pkg/openfeature, which has an unnecessary
pkg in the import path, to github.com/open-feature/go-sdk/openfeature,
without the unnecessary "pkg" in the import path.

The existing github.com/open-feature/go-sdk/pkg/openfeature package is
now a compatibility shell; exported types, constants, variables, and
functions are now aliased to the new
github.com/open-feature/go-sdk/openfeature equivalents in a way that
will pass equality checks, and types are interchangeable between both
packages. No logic or tests live in the package anymore, only the
exported identifiers, which call through to the new package for
behavior.

The memprovider package, similarly, has moved.

pkg/openfeature/api.go → openfeature/api.go

@@ -5,7 +5,7 @@ import (
 	"sync"
 
 	"github.com/go-logr/logr"
-	"github.com/open-feature/go-sdk/pkg/openfeature/internal"
+	"github.com/open-feature/go-sdk/openfeature/internal"
 	"golang.org/x/exp/maps"
 )