Previously we relied on CMake's export(PACKAGE ...) function when
finding dune packages without dune-control. This is error prone
when using several instances of a dune-module (either with differing
versions or built using different options). In this case there is no
control which of the instances is used.

Now we try to guess the correct build directory, if it was not
provided with ${module}_DIR, ${module}_Root or in the
CMAKE_PREFIX_PATH. Note that when using dunecontrol ${module}_DIR will
always be set. We take the path of the current toplevel build
directory and simply substitute any occurence of the name of current
module with name of the module that we try to find. This works for
both  building in a subdirectory of the source tree
(e.g. ${module-source}/build-cmake) or in a sibling directory