docs/gen-html-index: Make HTML::TreeBuilder::XPath optional again

docs/gen-html-index: Make HTML::TreeBuilder::XPath optional again

7782db9260d4 "docs/gen-html-index: Extract titles from HTML documents"
requires HTML::TreeBuilder::XPath.

This is sadly not as widely available as I had hoped.  Work around
this problem by making the use of this module optional: instead of
`use'ing at the toplevel, we `require' it in the eval.  If it's not
present, then the title is simply not extracted and the filename is
used as before, which is tolerable.

Also add some debugging.

Reported-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Tested-by: Doug Goldstein <cardoe@cardoe.com>