Move all files from pkg/openfeature to the root.

[paddycarver]

This PR

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.

Related Issues

Fixes #227

How to test

Any consumers of the package that already exist should be able to transparently use this version (go mod edit -replace github.com/open-feature/go-sdk=github.com/paddycarver/open-feature@root) with no code changes or changes in behavior.

[github-actions[bot]]

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