httplex was the original package name for shared code between net/http
and x/net/http2, but its name was too specific, and http/httpguts was
added later for other shared code.

We discussed merging httplex into httpguts at the time, but it didn't
happen earlier. This finishes the move.

Updates golang/go#23908

Change-Id: Ic7d6f39e584ca579d34b5ef5ec6a0c002a38a83c
Reviewed-on: https://go-review.googlesource.com/111875Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>