Fix a few nits in "opm-core-prereqs.cmake"

@andlaus suggested (https://github.com/OPM/opm-core/pull/503#discussion_r10197046) that the search for ERT be promoted ahead of "opm-parser" in opm-core's prerequisite file.  I agree, but chose instead to defer the search for "opm-parser" until after we've established the local state of ERT availability to mimic the order of other modules.

This change-set also explicitly states the role of opm-parser within the project.