Switch to built-in 'httpx._core' instead of 'httpcore'

[tomchristie]

See discussion #1741.

First pass at including the core networking functionality of httpcore, but built-in directly to httpx.

There's a bunch of work remaining on this, but wanted to get a first pass up for now.

The work here does some fundamental rejigging in order to:

• Ensure that we're able to properly test the networking with most tests being against mock streams.
• Deal with PoolTimeout when num tasks in asyncio.gather() exceeds client max_connections #1171
• Deal with NewConnectionRequired exception raised when running behind a proxy httpcore#335
• Enforce 100% coverage of the networking code.

• Wire-up HTTPTransport to httpx._core.ConnectionPool.
• Implement proxy support.
• Implement timeouts.
• Wire up correct Transport exceptions.
• AnyIO backend.
• HTTP/2.
• Retries, UDS, Local Address.

Everything we're working with here is private API, but the plan is that longer term we'll eventually open some of it up as public API.