Resolve symlinks in LOAD_PATH when activating pre-required default gems

Some double load issues were reported a while ago by OS packagers where
if a gem has been required before rubygems, and then after, rubygems
require would cause a double load.

We avoid this issue by activating the corresponding gem if we detect
that a file in the default LOAD_PATH that belongs to a default gem has
already been required when rubygems registers default gems.

However, the fix does not take into account that the default LOAD_PATH
could potentially include symlinks. This change fixes the same double
load issue described above but for situations where the default
LOAD_PATH includes symlinks.

lib/rubygems.rb

@@ -1293,7 +1293,12 @@ def already_loaded?(file)
     end
 
     def default_gem_load_paths
-      @default_gem_load_paths ||= $LOAD_PATH[load_path_insert_index..-1]
+      @default_gem_load_paths ||= $LOAD_PATH[load_path_insert_index..-1].map do |lp|
+        expanded = File.expand_path(lp)
+        next expanded unless File.exist?(expanded)
+
+        File.realpath(expanded)
+      end
     end
   end