[Fuchsia] Rename //fuchsia/net_http to //fuchsia/http.

This makes the directory layout consistent with the name of the
Fuchsia package it contains. In addition, this CL removes the C++
namespace in this part of the //fuchsia directory as this code is not
called from anywhere else. This CL also cleans up the build file,
using more common target names and setting the proper visibility to
targets.

TBR=jbudorick@chromium.org

Bug: 922635
Change-Id: I47f512e1bfb76dfca98261c884f4fec2da11a23d
Reviewed-on: https://chromium-review.googlesource.com/c/1423087
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624428}
19 files changed