Commit 6905e9ed885cd1d5be26106d64831a6d35c36bd9 permanently enabled the
on-disk cache in EDS for Google Contacts address books, so the option is now
redundant.
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.
