Bug#1018004: qt6-tools: does not build and link with system litehtml

[Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>]

Control: block 1018004 by 1018006

Hi Rob!

Thanks a lot for your patch! If possible, next time try to create a MR
in https://salsa.debian.org/qt-kde-team/qt6/qt6-tools/ , that makes it
much easier for us to review.

Now looking at the code the real issue is on litehtml itself. By
modifying src/assistant/qlitehtml/src/CMakeLists.txt to make a verbose
find_package():

```
CMake Warning at src/assistant/qlitehtml/src/CMakeLists.txt:15 (find_package):
 By not providing "Findlitehtml.cmake" in CMAKE_MODULE_PATH this
project has
 asked CMake to find a package configuration file provided by
"litehtml",
 but CMake did not find one.

 Could not find a package configuration file provided by "litehtml"
with any
 of the following names:

   litehtmlConfig.cmake
   litehtml-config.cmake

 Add the installation prefix of "litehtml" to CMAKE_PREFIX_PATH or set
 "litehtml_DIR" to a directory containing one of the above files.  If
 "litehtml" provides a separate development package or SDK, be sure it
has
 been installed.
```

Looking at liblitehtml-dev:

```
$ dpkg -L liblitehtml-dev | grep -i cmake
/usr/lib/x86_64-linux-gnu/cmake
/usr/lib/x86_64-linux-gnu/cmake/litehtml
/usr/lib/x86_64-linux-gnu/cmake/litehtml/litehtmlTargets-none.cmake
/usr/lib/x86_64-linux-gnu/cmake/litehtml/litehtmlTargets.cmake
```

it is not providing the right CMake files.

So the fix really lies on liblitehtml. qttools already has the right
code in order to make it work, liblitehtml needs to ship the right
cmake files.


-- 
Lisandro Damián Nicanor Pérez Meyer
https://perezmeyer.com.ar/