This commit relocates recently introduced web artifacts in the
TestContext framework to the ~.test.context.web package and renames some
classes for consistency with the existing code base.

 - introduced package-info.java in the web package.

 - ServletTestExecutionListener now extends
   AbstractTestExecutionListener instead of implementing
   TestExecutionListener.

 - relocated AbstractGenericWebContextLoader,
   AnnotationConfigWebContextLoader, XmlWebContextLoader, and
   WebDelegatingSmartContextLoader to the web package.

 - renamed XmlWebContextLoader to GenericXmlWebContextLoader for
   consistency with GenericXmlContextLoader.

 - changed the visibility of AbstractDelegatingSmartContextLoader and
   AnnotationConfigContextLoaderUtils to public.

Issue: SPR-10067