Otherwise implicit dependencies are never search for. That was the case
for dune-geometry and dune-grid when configuring opm-core as these are only
transitively required via opm-grid.
With this commit we include <module>-prereqs.cmake in <module>-config.cmake
and thus trigger searches for all modules that this module depends on.
We also install the file now <module>-prereqs.cmake as we need it for
installed modules, too.
use pure config mode.
we no longer compile a test application for the libraries,
however checks have been tightened by making sure version of
all opm modules are the same.
this allows for putting prereqs directly in the module repositories
