Move all internal traits and classes to zio.http.internal package

[jdegoes]

What is not easy to do right now?

Many internal implementation details of the library that should only ever be seen by maintainers or very advanced users are visible in the organizational details and generate Scaladoc.

Describe the solution you'd like

Move internal implementation details (including internal traits) to an internal package, with package-private access modifiers, so that only end-user data types appear in the public API.

Why is it important?

Improving user experience and consistency with ZIO Core.

NOTE: Much of these are currently in zio.http.services.

[senia-psm]

I want to take this.