Fix bugs in DIYStage: fetch & context handling.
Fix bugs in DIYStage: fetch & context handling.

- DIYStage needs to be a context handler
- DIYStage.fetch needs to take 2 args.

lgarren/spack,mfherbst/spack,krafczyk/spack,iulian787/spack,skosukhin/spack,LLNL/spack,TheTimmy/spack,mfherbst/spack,EmreAtes/spack,TheTimmy/spack,tmerrick1/spack,matthiasdiener/spack,tmerrick1/spack,krafczyk/spack,skosukhin/spack,krafczyk/spack,mfherbst/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,EmreAtes/spack,lgarren/spack,iulian787/spack,skosukhin/spack,TheTimmy/spack,mfherbst/spack,mfherbst/spack,lgarren/spack,iulian787/spack,lgarren/spack,matthiasdiener/spack,tmerrick1/spack,matthiasdiener/spack,TheTimmy/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,krafczyk/spack,EmreAtes/spack,LLNL/spack,krafczyk/spack,tmerrick1/spack,matthiasdiener/spack,iulian787/spack,TheTimmy/spack,matthiasdiener/spack,EmreAtes/spack,skosukhin/spack,skosukhin/spack