provide missing input properties for multiplex rules

baseName, completeBaseName and baseDir were only available for
simplex rule inputs. Now inputs and outputs for simplex and
multiplex rules have exactly the same interface.

This is a preparation for removing
RulesApplicator::setupScriptEngineForArtifact, which is the
place where we set up input objects for simplex rules.

Change-Id: I2cbe614ed734e79205c1595619312fcf028dc708
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>