Fixes setting of module path for installed package.

Previously the module path was a relative to the install
directory. Because of this dependant modules did not find
the macros of dune-common and cmake configure did not succeed.
With this the CMake package configuration file contains the
full path and the macros are found (even if there is no local
dune-common lying around).
[duneproject,release] Generate cmake/pkg/dune-module-config.cmake.in with full path to macros.

This is a follow up to the previous patch. It corrects the situation
for new modules created with duneproject.