Redirection loop in index.html

index.html is provided for browsers that use this file when given a URL for an enclosing directory. JavaDoc may set this up as a redirection to a more specific default, for a module, package, class or interface.

Under some circumstances, the redirection is broken, effectively pointing to itself, causing a browser to repeatedly reload the page.

This may be caused by HtmlConfiguration.setTopFile, which might (silently) not set the top file.

This was discovered while investigating JDK-8322865. For more details, see the comments there.

In particular, notice the initial short-circuit via `checkForDeprecation`, and possible fall through if no packages are found.