if the dune.module file is not found for a certain module, dunecontrol

tries to find an installed module via pkg-config. If pkg-config finds
an installed module, dunecontrol proceeds, although the module is
dropped from the internal tree, as installed modules don't need to be
built. If pkg-config does not find the module, an error is throughn.
If a dune.module file is found, installed versions of the module are
ignored.


[[Imported from SVN: r4741]]