Proposed fix for pkg_admin +REQUIRED_BY duplicates

[Jonathan Perkin <jperkin%joyent.com@localhost>]

Here's a patch that not only fixes duplicate entries in +REQUIRED_BY
files after running "pkg_admin rebuild-tree", but also improves
performance significantly:

  https://gist.github.com/jperkin/98550d5bd07f4179ebfeea825fc3ec20

tl;dr: reduces the runtime of "pkg_admin rebuild-tree" on a system
with 12,762 packages installed down from 13m 52s to just 1m 4s.  Even
on my local MacBook with only 197 packages it halves the runtime.

With this in place I will be changing pkgin to run rebuild-tree after
every upgrade, and won't be pursuing the other proposed change as it
doesn't handle all cases.

Thanks,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com