Change the installation path for E-D-S backends.
Address book and calendar backend modules are now split into different
installation directories so the D-Bus factory processes will only load
relevant backend modules.
This changes some pkg-config details for third-party backend modules.
Instead of querying the backend directory with:
pkg-config --variable=extensiondir evolution-data-server-1.2
you must query the directory for address book backends with:
pkg-config --variable=backenddir libedata-book-1.2
and the directory for calendar backends with:
pkg-config --variable=backenddir libedata-cal-1.2