Removing find-prefix infrastructure

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

Hi all,

I'd like to propose removing the mk/find-prefix.mk infrastructure and
related configuration.  This was required in a pkgviews world to find
the installation prefix of a pkgsrc package, but since we removed
pkgviews it is entirely redundant.

The diff is quite large, but is essentially replacing any FIND_PREFIX
usage with either:

  BUILDLINK_PREFIX.pkg if pulled in via buildlink3.mk
  LOCALBASE if pulled in via DEPENDS

The diff URL:

  https://github.com/joyent/pkgsrc/compare/trunk...joyent:joyent/review/find-prefix

I have performed two bulk builds, one without and one with this
change, they are for the most part identical (there was a typo in the
rpm2pkg path that I have since fixed):

  http://us-east.manta.joyent.com/pkgsrc/public/reports/upstream-bulktest/20150923.1210/meta/report.html
  http://us-east.manta.joyent.com/pkgsrc/public/reports/upstream-bulktest/20150930.1345/meta/report.html

Removing find-prefix also means a minor performance improvement, as we
no longer have to shell out to calculate the package prefix for each
package using the infrastructure.  For example, running:

  $ cd pkgsrc/audio/amarok-kde3
  $ ptime bmake show-var VARNAME=WRKOBJDIR

reduces from:

  real       19.713038895
  user       10.290976777
  sys         6.764893527

down to:

  real       18.291650722
  user        9.772795473
  sys         5.856298487

thanks to having to execute 81 fewer `pkg_info -qp` calls and
associated shell processes.

I'd like to commit this quite soon so that we have plenty of time to
address any potential fallout ready for the next branch, so please let
me know if you have any issues with this work.

Thanks,

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