Split exported targets in module and others

The following discussion from !1247 (merged) should be addressed:

• @simon.praetorius started a discussion: (+2 comments)

  I think here was a misunderstanding: "MODULE_LIBRARY" just means that it can be linked by default and is part of DUNE_LIBS, I think. But, if NO_MODULE_LIBRARY is set, we do not mean that it should not be exported, it just should not be linked by default to all targets. Example: dunealbertagridXd in dune-grid. This is excluded from the list of module libraries, because we don't know which dimensionworld should be used. This has to be selected by the user when calling add_dune_alberta_flags. Now, if it is not a module library, it is not put into the INTERFACE_LIBRARIES set and thus no unscoped targets are created. Thus, dune-grid alberta cannot be used in downstream target. This is just one example, where we can find a fix in dune-grid soon. But in other modules this might be more critical.

  We need to merge dune-grid!709 (merged) otherwise albertagrid cannot be used.