Add option to copy only certain deptypes to Spec.copy()
Add option to copy only certain deptypes to Spec.copy()

- can now pass these to Spec.copy() and Spec._dup():
  - deps=True
  - deps=False
  - deps=(list of deptypes)

- Makes it easy to filter out only part of a spec.

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