Buck: Simplify root build file

Buck: Simplify root build file

Remove all proxy rules, that were introduced to allow to build Gerrit
with hijacked JGit cell. New approach suggested in: [1], that emulates
real JGit project structure in its own cell, makes them unnecessary.

Add :all rule, that build all artifacts and packages them in zip file.
Add shell binary :jgit_bin rule, that allows to execute JGit binary
from with buck run command, e.g.:

  $ buck run jgit_bin status
  $ buck run jgit_bin -- --version

* [1] https://gerrit-review.googlesource.com/74859

Change-Id: Idf9ecb783cbd2b9984d4118047968f1f5204d642
Signed-off-by: David Ostrovsky <david@ostrovsky.org>