html-tidy: unify with its drop-in replacement tidy-html5

- the original project has been unmaintained for years
- some dependants needed to be patched due to renamed headers
  https://github.com/htacg/tidy-html5/issues/326#issuecomment-160329114
- separate tidy-html5 package was removed, as since the 5.0.0 version
  it's meant as a successor to both, and library name got back
  from libtidy5.so to libtidy.so
  https://github.com/htacg/tidy-html5/issues/326#issuecomment-160314666

/cc committers to tidy-html5: @edwjto and @zimbatm.