Mixedrealitytoolkit-unity: vNext Task Update Core namespaces

[davidkline-ms]

Overview

The SDK namespaces are all in the format: Microsoft.MixedReality.Toolkit.SDK..
Core namespaces are Microsoft.MixedReality.Toolkit.Internal..

To make our structure easier to understand, we should replace Internal with Core.

Requires

Search/replace of Internal with Core in namespaces throughout Core and SDK files.
Discussion re: changing assembly names to not include "internal"

Acceptance criteria

• [ ] Nothing breaks :)

[StephenHodgson]

Pre or post alpha?

[davidkline-ms]

Pre or post alpha?

I currently have it post-alpha to reduce churn as we wrap it up.

[StephenHodgson]

Let's also move the systems into the final locations we decide as well when we implement this change.

[davidkline-ms]

Starting on this now...