make plugin _dependencies.pri file optional

plugins that cannot be depended upon don't need it.
of course, the project file needs to directly define the respective
variables instead.

this makes it possible to have stand-alone plugins in arbitrarily named
directories.

Change-Id: I166d997bb4db41feac9ffe897cefce382e533bb0
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>