Added support for sys-bin and sys-python virtual dependencies

Benjamin Wingerrequested to merge
bmwinger/portmod:system_deps into master

Raising an exception may not be the best way of handling this down the road, but for now it's the best way of getting a nice message to the user. It will likely have to be reworked as part of #59 (closed).

Closes #65 (closed).

Edited by Benjamin Winger

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