Fixing the distinction between "find" and "use"

orocos_find_package() now only finds and sets individual ${${PACKAGE}_*}
variables, and orocos_use_package() now adds those variables to the
aggregated ${USE_OROCOS_*} variables. As such, the catkin package.xml
parsing now uses orocos_use_package() but with OROCOS_NO_AUTO_LINKING
enabled so that nothing gets auto-linked. This enables people to _find_
orocos packages withoug having other side-effects not related to that
package.