Improved warning for missing analyzer dependencies

By design, dependencies of analyzers must be specified explicitly to ensure that builds are consistent. However, this is surprising and the warning that is generated when a dependency is missing does not give any hints as to how the problem might be solved. I updated the warning to clearly explain what must be done to resolve it.

Old warning: "Analyzer assembly '{0}' depends on '{1}' but it was not found. Analyzers may not run correctly."
New warning: "Analyzer assembly '{0}' depends on '{1}' but it was not found. Analyzers may not run correctly unless the missing assembly is added as an analyzer reference as well."

Fixes #4057

src/VisualStudio/Core/Def/ServicesVSResources.resx

@@ -475,7 +475,7 @@ Use the dropdown to view and switch to other projects this file may belong to.</
     <value>MissingAnalyzerReference</value>
   </data>
   <data name="WRN_MissingAnalyzerReferenceMessage" xml:space="preserve">
-    <value>Analyzer assembly '{0}' depends on '{1}' but it was not found. Analyzers may not run correctly.</value>
+    <value>Analyzer assembly '{0}' depends on '{1}' but it was not found. Analyzers may not run correctly unless the missing assembly is added as an analyzer reference as well.</value>
   </data>
   <data name="SuppressionStateColumnHeader" xml:space="preserve">
     <value>Suppression State</value>