Allow to skip certain stages when dependencies are missing

If we add a new dataset, we want to run all the data processing steps, but for example skip the evaluation as we do not have labels yet. We still want to utilize the foreach functionality to iterate through our different datasets.

For this scenario, it would be helpful to have a kind of deps which specifies "if not present, skip the stage instead of throwing an error, but behave just as normal deps otherwise". Currently, we have to add dummy files manually in order to be able to run dvc repro.

We can use dvc repro --keep-going for now, but this does not differentiate between missing dependencies and other errors that might occur. Also, in cases like my example above, we want to treat the current state of the pipeline as clean.

Start by tracing how dvc repro handles dependencies, missing files, and foreach stages. Define the behavior so missing dependencies can skip a stage, ordinary dependency failures still error, iteration remains supported, and the resulting pipeline is considered clean.