[release,cmake] Install data files to package specific datadir like with automake

E.g. the files in am are now installed to share/am/dune-module instead of share/am.

am/CMakeLists.txt

@@ -10,5 +10,5 @@ install(PROGRAMS
   sourcescheck
   top-rules
   webstuff
-  DESTINATION ${CMAKE_INSTALL_DATADIR}/am
+  DESTINATION ${CMAKE_INSTALL_DATADIR}/dune-common/am
 )