A few improvements to YARPConfig and related files

YARP_LIBRARIES now contains only the libraries that the user is supposed to use directly, in order to avoid linking all the plugins, yarpcar, yarpmod etc.

YARPDependencies.cmake was renamed to YARPTarget.cmake (as you usually find in other libraries)

The libraries are now exported using the "YARP::" namespace, so in order to link them from outside YARP you can use YARP_LIBRARIES or
```cmake
  target_link_library(foo YARP::YARP_OS YARP::YARP_init)
```
Linking "YARP_OS YARP_init" is deprecated by commit 22421a0 and does no longer work.

**WARNING**: This pull request breaks iCub in a couple of places and possibly other stuff (wherever YARP libraries are linked by name and not using YARP_LIBRARIES), and it is not backwards compatible. **DO NOT MERGE IT**, but eventually just give me the ok, I will merge this and push the commit in iCub at the same time.