References of the wpkg code

At this point all the libraries (libdebpackages, libutf8, and libexpr) are all documented within the same wpkg reference. It even includes the tools.

This gives doxygen the possibility to fully link and document all the references from the libdebpackages to the other two libraries.

However, the documentation from the external libraries (libtld, bzip2, zlib, controlled_vars) are not documented on this site.