Use realpath in clean_load_path

what happens next will shock you:
```
From: /real_path/.rvm/gems/ruby-2.5.1/gems/bundler-1.16.2/lib/bundler/shared_helpers.rb @ line 339 Bundler::SharedHelpers#clean_load_path:

    330: def clean_load_path
    331:   # handle 1.9 where system gems are always on the load path
    332:   return unless defined?(::Gem)
    333:
    334:   bundler_lib = bundler_ruby_lib
    335:
    336:   loaded_gem_paths = Bundler.rubygems.loaded_gem_paths
    337:
    338:   binding.pry
 => 339:   $LOAD_PATH.reject! do |p|
    340:     path = File.expand_path(p)
    341:     path = File.realpath(path) if File.exist?(path)
    342:     next if path.start_with?(bundler_lib)
    343:     loaded_gem_paths.delete(p)
    344:   end
    345:   $LOAD_PATH.uniq!
    346: end

[1] pry(#<Bundler::Runtime>)> bundler_lib
=> "/real_path/.rvm/gems/ruby-2.5.1/gems/bundler-1.16.2/lib"
[2] pry(#<Bundler::Runtime>)> loaded_gem_paths
=> ["/symlinked_path/.rvm/gems/ruby-2.5.1@global/gems/did_you_mean-1.2.0/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/bundler-1.16.2/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/byebug-10.0.2/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/extensions/x86_64-linux/2.5.0/byebug-10.0.2",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/coderay-1.1.2/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/method_source-0.9.0/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/pry-0.11.3/lib",
 "/symlinked_path/.rvm/gems/ruby-2.5.1/gems/pry-byebug-3.6.0/lib",
 "/real_path/.rvm/rubies/ruby-2.5.1/lib/ruby/gems/2.5.0/gems/fileutils-1.0.2/lib",
 "/real_path/.rvm/rubies/ruby-2.5.1/lib/ruby/gems/2.5.0/gems/psych-3.0.2/lib",
 "/real_path/.rvm/rubies/ruby-2.5.1/lib/ruby/gems/2.5.0/extensions/x86_64-linux/2.5.0/psych-3.0.2"]
```

basically we're not rejecting `bundler_lib` because symlink there and real path in $LOAD_PATH

Fixes rubygems#6465

lib/bundler/shared_helpers.rb

@@ -336,7 +336,12 @@ def clean_load_path
       loaded_gem_paths = Bundler.rubygems.loaded_gem_paths
 
       $LOAD_PATH.reject! do |p|
-        next if File.expand_path(p).start_with?(bundler_lib)
+        expanded_p = File.expand_path(p)
+        next if expanded_p.start_with?(bundler_lib)
+        if File.exist?(expanded_p) && File.respond_to?(:realpath)
+          path = File.realpath(expanded_p)
+          next if path.start_with?(bundler_lib)
+        end
         loaded_gem_paths.delete(p)
       end
       $LOAD_PATH.uniq!

spec/runtime/setup_spec.rb

@@ -857,6 +857,49 @@ def clean_load_path(lp)
     expect(out).to eq("true\ntrue")
   end
 
+  context "with bundler is located in symlinked GEM_HOME" do
+    let(:gem_home) { Dir.mktmpdir }
+    let(:symlinked_gem_home) { Tempfile.new("gem_home") }
+    let(:bundler_dir) { File.expand_path("../..", __FILE__) }
+    let(:bundler_lib) { File.join(bundler_dir, "lib") }
+    before do
+      FileUtils.ln_sf(gem_home, symlinked_gem_home.path)
+      gems_dir = File.join(gem_home, "gems")
+      specifications_dir = File.join(gem_home, "specifications")
+      Dir.mkdir(gems_dir)
+      Dir.mkdir(specifications_dir)
+
+      FileUtils.ln_s(bundler_dir, File.join(gems_dir, "bundler-#{Bundler::VERSION}"))
+
+      gemspec = File.read("#{bundler_dir}/bundler.gemspec").
+                sub("Bundler::VERSION", %("#{Bundler::VERSION}"))
+      gemspec = gemspec.lines.grep_v(%r{lib/bundler/version}).join
+
+      File.open(File.join(specifications_dir, "bundler.gemspec"), "wb") do |f|
+        f.write(gemspec)
+      end
+    end
+
+    it "should succesfully require 'bundler/setup'" do
+      install_gemfile ""
+
+      ENV["GEM_PATH"] = symlinked_gem_home.to_path
+
+      ruby <<-R
+        if $LOAD_PATH.include?("#{bundler_lib}")
+          # We should use bundler from GEM_PATH for this test, so we should
+          # remove path to the bundler source tree
+          $LOAD_PATH.delete("#{bundler_lib}")
+        else
+          raise "We don't have #{bundler_lib} in $LOAD_PATH"
+        end
+        puts (require 'bundler/setup')
+      R
+
+      expect(out).to eql("true")
+    end
+  end
+
   it "stubs out Gem.refresh so it does not reveal system gems" do
     system_gems "rack-1.0.0"