Refactor Bundler::Dsl#check_rubygems_source_safety to improve readabi…

…lity

`check_rubygems_source_safety` is responsible for:

1. if there are multiple global sources
  - for bundle 3.x raise an error
  - for bundle 2.x print a warning
2. print a warning if there is no explicit global source

The second responsibility was added recently and now the logic could be
extracted to improve readability. Conditions are still live in the `check_rubygems_source_safety` method
since we don't want to call both functions always and that would help us achieve that.

bundler/lib/bundler/dsl.rb

@@ -448,14 +448,20 @@ def check_path_source_safety
 
     def check_rubygems_source_safety
       if @sources.implicit_global_source?
-        Bundler::SharedHelpers.major_deprecation 2, "This Gemfile does not include an explicit global source. " \
-          "Not using an explicit global source may result in a different lockfile being generated depending on " \
-          "the gems you have installed locally before bundler is run." \
-          "Instead, define a global source in your Gemfile like this: source \"https://rubygems.org\"."
+        implicit_global_source_warning
+      elsif @sources.aggregate_global_source?
+        multiple_global_source_warning
       end
+    end
 
-      return unless @sources.aggregate_global_source?
+    def implicit_global_source_warning
+      Bundler::SharedHelpers.major_deprecation 2, "This Gemfile does not include an explicit global source. " \
+        "Not using an explicit global source may result in a different lockfile being generated depending on " \
+        "the gems you have installed locally before bundler is run." \
+        "Instead, define a global source in your Gemfile like this: source \"https://rubygems.org\"."
+    end
 
+    def multiple_global_source_warning
       if Bundler.feature_flag.bundler_3_mode?
         msg = "This Gemfile contains multiple primary sources. " \
           "Each source after the first must include a block to indicate which gems " \

bundler/spec/bundler/dsl_spec.rb

@@ -245,7 +245,7 @@
   describe "#check_primary_source_safety" do
     context "when a global source is not defined implicitly" do
       it "will raise a major deprecation warning" do
-        not_a_global_source = double("not-a-global-source", no_remotes?: true, multiple_remotes?: false)
+        not_a_global_source = double("not-a-global-source", :no_remotes? => true)
         allow(Bundler::Source::Rubygems).to receive(:new).and_return(not_a_global_source)
 
         warning = "This Gemfile does not include an explicit global source. " \