Move all files from pkg/openfeature to the root.

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, 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
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.

The shortcoming of this import path is that goimports will struggle in
some situations to be able to automatically add the import path based on
the package name when it's used for the first time in a consumer
package.

pkg/openfeature/api.go → 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/internal"
 	"golang.org/x/exp/maps"
 )