Rewrite the handling of the local kdoctools sources

Remove the hack which used to rely on XDG_DATA_DIRS, which
does not seem to work with kdoctools or most likely the Qt
version in bionic (maybe some QStandardPath change?)
and simply rely on the --srcdir meinproc parameter,
which seems to work for meinproc.
xml2po use a different code and does't care about the
entities and the DTD, apparently.