Remove version overriding stuff unless necessary

By doing this we avoid circular requires (`rubygems` requires `bundler` via
`USE_GEMDEPS`, `bundler` requires `rubygems` when loading its own version), and
also redefinition warnings when the `bundler.gemspec` is evaluated with
`rubygems` already loaded (for example, when running `ruby setup.rb`
from `rubygems` repo).

When working with rubygems versions that didn't have the bundler version
finder, it seems to be necessary, but for newer rubygems versions it's
not.

lib/bundler/rubygems_integration.rb

@@ -317,6 +317,10 @@ def stubs_provide_full_functionality?
       provides?(">= 2.5.2")
     end
 
+    def has_bundler_version_finder?
+      provides?(">= 2.7.0")
+    end
+
     def replace_gem(specs, specs_by_name)
       reverse_rubygems_kernel_mixin
 

lib/bundler/version.rb

@@ -1,23 +1,29 @@
 # frozen_string_literal: false
 
+require_relative "rubygems_integration"
+
 module Bundler
-  # We're doing this because we might write tests that deal
-  # with other versions of bundler and we are unsure how to
-  # handle this better.
-  VERSION = "2.1.0.pre.1".freeze unless defined?(::Bundler::VERSION)
+  if Bundler.rubygems.has_bundler_version_finder?
+    VERSION = "2.1.0.pre.1".freeze
+  else
+    # We're doing this because we might write tests that deal
+    # with other versions of bundler and we are unsure how to
+    # handle this better.
+    VERSION = "2.1.0.pre.1".freeze unless defined?(::Bundler::VERSION)
 
-  def self.overwrite_loaded_gem_version
-    begin
-      require "rubygems"
-    rescue LoadError
-      return
+    def self.overwrite_loaded_gem_version
+      begin
+        require "rubygems"
+      rescue LoadError
+        return
+      end
+      return unless bundler_spec = Gem.loaded_specs["bundler"]
+      return if bundler_spec.version == VERSION
+      bundler_spec.version = Bundler::VERSION
     end
-    return unless bundler_spec = Gem.loaded_specs["bundler"]
-    return if bundler_spec.version == VERSION
-    bundler_spec.version = Bundler::VERSION
+    private_class_method :overwrite_loaded_gem_version
+    overwrite_loaded_gem_version
   end
-  private_class_method :overwrite_loaded_gem_version
-  overwrite_loaded_gem_version
 
   def self.bundler_major_version
     @bundler_major_version ||= VERSION.split(".").first.to_i