Fix jruby warnings

On jruby, somehow the $LOAD_PATH is manipulated so that we end up
requiring stuff inside the built package even if we have specified the
-I flag, so we get redefinition warnings.

I'm not sure about the root cause, but relative requiring fixes it, and
it's faster anyways.

Author: deivid-rodriguez

lib/rubygems/command.rb

@@ -6,8 +6,8 @@
 #++
 
 require 'optparse'
-require 'rubygems/requirement'
-require 'rubygems/user_interaction'
+require_relative 'requirement'
+require_relative 'user_interaction'
 
 ##
 # Base class for all Gem commands.  When creating a new gem command, define

lib/rubygems/ext.rb

@@ -10,9 +10,9 @@
 
 module Gem::Ext; end
 
-require 'rubygems/ext/build_error'
-require 'rubygems/ext/builder'
-require 'rubygems/ext/configure_builder'
-require 'rubygems/ext/ext_conf_builder'
-require 'rubygems/ext/rake_builder'
-require 'rubygems/ext/cmake_builder'
+require_relative 'ext/build_error'
+require_relative 'ext/builder'
+require_relative 'ext/configure_builder'
+require_relative 'ext/ext_conf_builder'
+require_relative 'ext/rake_builder'
+require_relative 'ext/cmake_builder'

lib/rubygems/ext/build_error.rb

@@ -2,7 +2,7 @@
 ##
 # Raised when there is an error while building extensions.
 
-require 'rubygems/exceptions'
+require_relative '../exceptions'
 
 class Gem::Ext::BuildError < Gem::InstallError
 end

lib/rubygems/ext/builder.rb

@@ -5,7 +5,7 @@
 # See LICENSE.txt for permissions.
 #++
 
-require 'rubygems/user_interaction'
+require_relative '../user_interaction'
 
 class Gem::Ext::Builder
 

lib/rubygems/ext/cmake_builder.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require 'rubygems/command'
+require_relative '../command'
 
 class Gem::Ext::CmakeBuilder < Gem::Ext::Builder
 

lib/rubygems/specification_policy.rb

@@ -420,7 +420,7 @@ def validate_removed_attributes # :nodoc:
   end
 
   def validate_extensions # :nodoc:
-    require 'rubygems/ext'
+    require_relative 'ext'
     builder = Gem::Ext::Builder.new(@specification)
 
     rake_extension = @specification.extensions.any? {|s| builder.builder_for(s) == Gem::Ext::RakeBuilder }