Certain interfaces and components are not exported, yet essential

[Dalimil]

1. The only way to import CalloutContentBase is like this:
   import { CalloutContentBase } from 'office-ui-fabric-react/lib/components/Callout/CalloutContent.base';
2. The only way to import PanelBase is like this:
   import { PanelBase } from 'office-ui-fabric-react/lib/components/Panel/Panel.base';
3. Same for IButtonClassNames
4. Same for IContextualMenuClassNames
5. Same for IMenuItemClassNames
6. Same for test utilities expectOne and expectMissing

All of these should properly export these, otherwise deep imports (officially unsupported) are unavoidable.
This blocks transition from office-online-fabric-react to fluentui/react

[ecraig12345]

If you need the test utilities, those should probably be added to @fluentui/test-utilities and exported from there. (Not sure why that wasn't done to begin with.)

[paulgildea]

@Dalimil Thanks for filing this issue.

@behowell / @JustSlone Can we coordinate with @Dalimil to understand their deep import needs and determine the best way forward. I noticed a some dependencies on base components, which leads me to believe that the components Panel and Callout might be missing something. If so let's see if we can contribute something in.

[ecraig12345]

@Dalimil already submitted a PR fixing most of these (thanks!) and we're discussing there.