Ensure we also install the WorldBuilder header files.

[bangerth]

#6450 is about the fact that we don't tell plugins where to find the WorldBuilder header files. We missed that because nobody apparently had been referencing WorldBuilder header files directly in their plugin descriptions.

But even when that is fixed, there is the issue that in our build system, we are referencing WorldBuilder header files in the WorldBuilder source directory. This won't work for the plugins -- they need to reference either the ones in the ASPECT build directory or the install directory. In other words, we need to make sure that we also install the WorldBuilder header files, not just our own. I must admit that I don't know how to achieve that, so won't fix it right away, but it's clearly wrong.

Alternatively, of course, we could just say that if a plugin wants to reference WorldBuilder header files, they are on their own and need to write the necessary CMake logic.