Fix package docnames for select return types, and some more docgen refactoring.

pkg/docgen/test/multi_library_test.dart

 library single_library_test;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import '../lib/docgen.dart';
 
 const String DART_LIBRARY_1 = '''
@@ skipping 110 matching lines @@
           // Testing Doc comment for class [B].
           var libraryMirror = mirrorSystem.libraries[testLibraryUri];
           var classDocComment = library.fixReference('B').children.first.text;
           expect(classDocComment, 'testLib.B');
 
           // Test for linking to parameter [c]
           var importedLib = libraryMirror.libraryDependencies.firstWhere(
             (dep) => dep.isImport).targetLibrary;
           var aClassMirror = importedLib.classes.values.first;
           expect(aClassMirror.qualifiedName, 'testLib2.foo.B');
-          var exportedClass = getDocgenObject(aClassMirror, library);
+          var exportedClass = Indexable.getDocgenObject(aClassMirror, library);
           expect(exportedClass is Class, isTrue);
 
 
           var method = exportedClass.methods['doThis'];
           expect(method is Method, isTrue);
           var methodParameterDocComment = method.fixReference(
               'c').children.first.text;
           expect(methodParameterDocComment, 'testLib.B.doThis.c');
 
 
           expect(method.fixReference('A').children.first.text, 'testLib.A');
           // Testing trying to refer to doThis function
           expect(method.fixReference('doThis').children.first.text,
               'testLib.B.doThis');
 
           // Testing trying to refer to doThis function
           expect(method.fixReference('doElse').children.first.text,
               'testLib.B.doElse');
 
 
           // Test a third library referencing another exported library in a
           // separate file.
           importedLib = libraryMirror.libraryDependencies.firstWhere(
             (dep) => dep.isImport && dep.targetLibrary.qualifiedName ==
             'testLib.bar').targetLibrary;
           aClassMirror = importedLib.classes.values.first;
           expect(aClassMirror.qualifiedName, 'testLib.bar.C');
-          exportedClass = getDocgenObject(aClassMirror, library);
+          exportedClass = Indexable.getDocgenObject(aClassMirror, library);
           expect(exportedClass is Class, isTrue);
           expect(exportedClass.docName, 'testLib.C');
 
           methodParameterDocComment = exportedClass.fixReference(
               'B').children.first.text;
           expect(methodParameterDocComment, 'testLib.B');
 
           methodParameterDocComment = exportedClass.fixReference(
               'testFunc').children.first.text;
           expect(methodParameterDocComment, 'testLib.testFunc');
 
         })).whenComplete(() => TEMP_DIRNAME.deleteSync(recursive: true));
     });
   });
 }