Remove fuchsia_sdk:fdio from public_deps in //base

1. Moved fdio to deps instead of public_deps in //base
2. Added direct deps on FDIO in targets that need it, particularly in
   mojo.
3. Also replaces some deps with public_deps in
   //mojo/edk/test:test_support to reflect set of header deps in the
   headers exported by that target.
4. Small cleanups in mojo/edk/BUILD.gn to make it more readable.
5. Updated build/config/fuchsia/BUILD.gn to avoid dependency on
   internals of third_party/fuchsia-sdk/BUILD.gn.

Bug: 707030
Change-Id: I402d3cd0d1d709aa381fa10e6fc6dfdb8e7e060c
Reviewed-on: https://chromium-review.googlesource.com/1052608
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#557935}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2293c96db6d66f2b49c48eab9335ba665827d283