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.

pkgs/development/libraries/mailcore2/default.nix

@@ -23,7 +23,9 @@ stdenv.mkDerivation rec {
     substituteInPlace CMakeLists.txt \
        --replace "tidy/tidy.h" "tidy.h" \
        --replace "/usr/include/tidy" "${libtidy}/include" \
-       --replace "/usr/include/libxml2" "${libxml2}/include/libxml2" \
+       --replace "/usr/include/libxml2" "${libxml2}/include/libxml2"
+    substituteInPlace src/core/basetypes/MCHTMLCleaner.cpp \
+      --replace buffio.h tidybuffio.h
   '';
 
   cmakeFlags = [