Auto merge of #5260 - bundler:seg-require-speed, r=indirect

Improve -rbundler performance

Along with https://github.com/rubygems/rubygems/pull/1801, speeds up `-rbundler` by ~5x

Profiled via

```ruby
# frozen_string_literal: true

REQUIRE_TIMES = {} # rubocop:disable Style/MutableConstant
module Kernel
  require "benchmark"
  alias_method :require_before_req, :require
  def require(path)
    ret = nil
    Benchmark.realtime do
      ret = require_before_req(path)
    end.tap do |t|
      if ret
        from = begin
                 caller[2]
               rescue
                 nil
               end
        ::REQUIRE_TIMES["#{path} : #{from}"] ||= t
      end
    end
    ret
  end
end

at_exit { puts REQUIRE_TIMES.to_a.sort_by(&:last).reverse.map {|k, v| "#{k} =>\n\t#{v}" } }
```

4 files changed, 16 insertions, 14 deletions

diff --git a/lib/bundler.rb b/lib/bundler.rb
index 0dcaa03965..9b96075ca9 100644
--- a/lib/bundler.rb
+++ b/lib/bundler.rb
@@ -7,7 +7,6 @@ require "tmpdir"
 
 require "bundler/errors"
 require "bundler/environment_preserver"
-require "bundler/gem_remote_fetcher"
 require "bundler/plugin"
 require "bundler/rubygems_ext"
 require "bundler/rubygems_integration"
@@ -32,6 +31,7 @@ module Bundler
   autoload :FeatureFlag,            "bundler/feature_flag"
   autoload :GemHelper,              "bundler/gem_helper"
   autoload :GemHelpers,             "bundler/gem_helpers"
+  autoload :GemRemoteFetcher,       "bundler/gem_remote_fetcher"
   autoload :GemVersionPromoter,     "bundler/gem_version_promoter"
   autoload :Graph,                  "bundler/graph"
   autoload :Index,                  "bundler/index"
diff --git a/lib/bundler/installer/parallel_installer.rb b/lib/bundler/installer/parallel_installer.rb
index 12f11bb5f0..c973233d99 100644
--- a/lib/bundler/installer/parallel_installer.rb
+++ b/lib/bundler/installer/parallel_installer.rb
@@ -88,6 +88,10 @@ module Bundler
     end
 
     def call
+      # Since `autoload` has the potential for threading issues on 1.8.7
+      # TODO:  remove in bundler 2.0
+      require "bundler/gem_remote_fetcher" if RUBY_VERSION < "1.9"
+
       enqueue_specs
       process_specs until @specs.all?(&:installed?) || @specs.any?(&:failed?)
       handle_error if @specs.any?(&:failed?)
diff --git a/lib/bundler/plugin/api.rb b/lib/bundler/plugin/api.rb
index bd7cef5382..a2d5cbb4ac 100644
--- a/lib/bundler/plugin/api.rb
+++ b/lib/bundler/plugin/api.rb
@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-require "bundler/plugin/api/source"
 
 module Bundler
   # This is the interfacing class represents the API that we intend to provide
@@ -24,6 +23,8 @@ module Bundler
   # and hooks).
   module Plugin
     class API
+      autoload :Source, "bundler/plugin/api/source"
+
       # The plugins should declare that they handle a command through this helper.
       #
       # @param [String] command being handled by them
diff --git a/lib/bundler/shared_helpers.rb b/lib/bundler/shared_helpers.rb
index 62ade29e30..613609f25f 100644
--- a/lib/bundler/shared_helpers.rb
+++ b/lib/bundler/shared_helpers.rb
@@ -8,6 +8,7 @@ require "bundler/current_ruby"
 
 module Gem
   class Dependency
+    # This is only needed for RubyGems < 1.4
     unless method_defined? :requirement
       def requirement
         version_requirements
@@ -18,8 +19,6 @@ end
 
 module Bundler
   module SharedHelpers
-    attr_accessor :gem_loaded
-
     def default_gemfile
       gemfile = find_gemfile
       raise GemfileNotFound, "Could not locate Gemfile" unless gemfile
@@ -217,22 +216,20 @@ module Bundler
     def bundler_ruby_lib
       File.expand_path("../..", __FILE__)
     end
-    private :bundler_ruby_lib
 
     def clean_load_path
       # handle 1.9 where system gems are always on the load path
-      if defined?(::Gem)
-        me = File.expand_path("../../", __FILE__)
-        me = /^#{Regexp.escape(me)}/
+      return unless defined?(::Gem)
 
-        loaded_gem_paths = Bundler.rubygems.loaded_gem_paths
+      bundler_lib = bundler_ruby_lib
 
-        $LOAD_PATH.reject! do |p|
-          next if File.expand_path(p) =~ me
-          loaded_gem_paths.delete(p)
-        end
-        $LOAD_PATH.uniq!
+      loaded_gem_paths = Bundler.rubygems.loaded_gem_paths
+
+      $LOAD_PATH.reject! do |p|
+        next if File.expand_path(p).start_with?(bundler_lib)
+        loaded_gem_paths.delete(p)
       end
+      $LOAD_PATH.uniq!
     end
 
     def prints_major_deprecations?