EBookSourceConfig drags in no additional dependencies, and allows us to
delay publishing a libevolution-addressbook.so since 3rd party packages
will need to subclass EBookSourceConfig.
The address book source code will need to be flattened into a single
library before we could publish a libevolution-addressbook.so anyway.
That would be a good thing to do regardless -- Evolution has way too
many internal libraries -- but it's out of scope at the moment.
