Rename YARP_OS -> YARP_os

[drdanz]

All the libraries have the name lowercase, but for historical reasons, YARP_OS is uppercase.
This might be a good moment to change it, therefore I suggest to rename the library to YARP_os, and to add ALIAS targets for YARP_OS.

As an alternate option (I'm not really sure if this can be done), we could have YARP_os as a normal library, and YARP_OS as an library that links both YARP_os and YARP_init.

Comments?

[traversaro]

My secret CMake dream is to have an ALIAS target that can generate a warning when linked, to deprecate easily old target names.

[drdanz]

I totally agree and was thinking exactly the same thing while I was opening this issue.
Opened a related feature request in CMake https://gitlab.kitware.com/cmake/cmake/issues/17824 😁