Refactor namespace Microsoft.Extensions.Internal

The following classes need to be public so that the code generator tool does not need any special treatment.

```C#
public class DotnetToolDispatcher { … }
public class Command { … }
```

The special privileges granted to the generator tool should be removed:
```C#
[assembly:InternalsVisibleTo("dotnet-aspnet-codegenerator, PublicKey=…")]

```

Also, the namespace ~`Microsoft.Extensions.Internal`~ does not really mean anything. It should be removed and its contents moved elsewhere.