[cmake] Do not use find_package in config mode for opm-parser.

Currently opm-parser does not create opm-parser-config.cmake files. Therefore
building with duncontrol fails as it sets opm-parser_DIR. With this patch
we force CMake to always use module mode for finding which fixes this.

cmake/Modules/OpmFind.cmake

@@ -59,6 +59,7 @@ set (_opm_proj_exemptions
   dune-istl
   dune-grid
   dune-geometry
+  opm-parser
   )
 
 # although a DUNE module, it is delivered in the OPM suite