require 'pp' before use PP

`Kernel#pp` has wrapper, but `PP` does not.

https://github.com/ruby/ruby/runs/207405029#step:10:141
`NameError: uninitialized constant Bundler::Molinillo::Resolver::Resolution::PP`

lib/bundler/vendor/molinillo/lib/molinillo/resolution.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+require 'pp'
 
 module Bundler::Molinillo
   class Resolver

lib/rubygems/resolver.rb

@@ -125,7 +125,10 @@ class Gem::Resolver
 
     data = yield
     $stderr.printf "%10s (%d entries)\n", stage.to_s.upcase, data.size
-    PP.pp data, $stderr unless data.empty?
+    unless data.empty?
+      require 'pp'
+      PP.pp data, $stderr
+    end
   end
 
   ##