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
Previously we assumed it to be ecl (like the project name).
That is not correct. With this commit we now use the correct
default clone directory, libecl, in the sibling search.
Even though people are telling that ecl is already found using
sibling search it turned out that this statement is false. It
was always found using the CMake package cache. When installing
this lead to the installed package using ecl from the build tree.
With this commit we first try to find ecl without the package cache
but maybe using sibling search. Building installed packages no works
by setting -DSIBLING_SEARCH=Off -DCMAKE_INSTALL_PREFIX=/install/path
