BuildDiff: cleanup imports and make slightly more readable

Dependencies

• [2] NYIIFA32 BuildDiff: split out from JobsetEval GET impl
• [3] 4CBVJEIS BuildDiff: move cmdBuilds from Controller/JobsetEval
• [4] XJN33EVY BuildDiff: make actual hash ref

Change contents

• edit in src/lib/Hydra/Helper/BuildDiff.pm at line 6
  [2.139]→[2.139:275](∅→∅)
  − use base 'Hydra::Base::Controller::NixChannel';

  − use Hydra::Helper::Nix;

  − use Hydra::Helper::CatalystUtils;

  − use List::SomeUtils qw(uniq);
• replacement in src/lib/Hydra/Helper/BuildDiff.pm at line 65
  [2.1890]→[2.1890:2007](∅→∅)
  − push @{$ret->{removed}}, { job => $build2->get_column('job'), system => $build2->get_column('system') };
  [2.1890]

  [2.2007]
  + my $job_system = { job => $build2->get_column('job'), system => $build2->get_column('system') };

  + push @{$ret->{removed}}, $job_system;