cmGeneratorTarget: Implement cmTargetPropertyComputer interface

Populate a local member for the sources property when the instance is
created.  Pass the parameter to avoid the policy check when doing so.
Ordinarily, the GetSources function should not be called unconditionally
(at generate time), but we need to do so here in case the property is
read in a generator expression.  The intent is to be able to implement
cmGeneratorTarget without requiring cmTarget.