Install default and bundled gems into dedicated directories

Previously, all default/bundled gems were installed into single location
such as `/usr/local/lib/ruby/gems/3.3.0+0`. However, the same location
is used for user installed gems, which is not optimal.

With this change, the default gems goes into
`/usr/local/lib/ruby/default_gems/3.3.0+0` while the bundled gems goes
into `/usr/local/lib/ruby/bundled_gems/3.3.0+0`. User installed gems
continues to land in `/usr/local/lib/ruby/gems/3.3.0+0`.

lib/rubygems/defaults/ruby.rb

@@ -0,0 +1,37 @@
+module Gem
+  class << self
+
+    def bundled_gems_dir
+      File.join(RbConfig::CONFIG["rubylibprefix"], "bundled_gems", RbConfig::CONFIG["ruby_version"])
+    end
+
+    def default_gems_dir
+      File.join(RbConfig::CONFIG["rubylibprefix"], "default_gems", RbConfig::CONFIG["ruby_version"])
+    end
+
+    undef :default_specifications_dir if method_defined? :default_specifications_dir
+
+    if method_defined? :default_path
+      alias orig_default_path default_path
+      undef :default_path
+    end
+
+    ##
+    # Path to specification files of default gems.
+
+    def default_specifications_dir
+      @default_specifications_dir ||= File.join(Gem.default_gems_dir, "specifications", "default")
+    end
+
+    ##
+    # Default gem load path
+
+    def default_path
+      path = orig_default_path
+      path << bundled_gems_dir
+      path << default_gems_dir
+      path
+    end
+
+  end
+end

tool/rbinstall.rb

@@ -942,7 +942,7 @@ def load_gemspec(file, base = nil)
 end
 
 def install_default_gem(dir, srcdir, bindir)
-  gem_dir = Gem.default_dir
+  gem_dir = Gem.default_gems_dir
   install_dir = with_destdir(gem_dir)
   prepare "default gems from #{dir}", gem_dir
   RbInstall.no_write do
@@ -991,7 +991,7 @@ def install_default_gem(dir, srcdir, bindir)
 end
 
 install?(:ext, :comm, :gem, :'bundled-gems') do
-  gem_dir = Gem.default_dir
+  gem_dir = Gem.bundled_gems_dir
   install_dir = with_destdir(gem_dir)
   prepare "bundled gems", gem_dir
   RbInstall.no_write do