[Fix] Make source-target workflow visible for `SaQC._planned`

Currently the source-target workflow is implemented outside of the evaluate method, that means that the implicit call to tools.copy is not stored in SaQC._computed and therefore not visible for consumers of that list (currently only the translators). The MR fixes this, by placing the copy operations into the main SaQC evaluation mechanism.

I think, this should be unproblematic, but honestly, I am not entirely sure. Maybe you want to have a look @palmb?