Implement Bundler::SourceList#implicit_global_source?

This method is created to tell whether any global source exist in the object or not and it will be used by `Bundler:Dsl` to print a warning if no global source has been defined in the Gemfile.

bundler/lib/bundler/source_list.rb

@@ -37,6 +37,10 @@ def aggregate_global_source?
       global_rubygems_source.multiple_remotes?
     end
 
+    def implicit_global_source?
+      global_rubygems_source.no_remotes?
+    end
+
     def add_path_source(options = {})
       if options["gemspec"]
         add_source_to_list Source::Gemspec.new(options), path_sources

bundler/spec/bundler/source_list_spec.rb

@@ -441,4 +441,19 @@
       source_list.remote!
     end
   end
+
+  describe "implicit_global_source?" do
+    context "when a global rubygem source provided" do
+      it "returns a falsy value" do
+        source_list.add_global_rubygems_remote("https://rubygems.org")
+
+        expect(source_list.implicit_global_source?).to be_falsey
+      end
+    end
+    context "when no global rubygem source provided" do
+      it "returns a truthy value" do
+        expect(source_list.implicit_global_source?).to be_truthy
+      end
+    end
+  end
 end