update for new cacheddownloader interface/fake

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
always extract in download; cache .tar files

this drastically simplifies download action, and pushes the archive
conversion heavy-lifting up to the cache, where it happens once per
cache key rather than N writes to disk per every download for the
conversion.

[#80392924]

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
wip

Signed-off-by: Nick Wade <nwade@pivotal.io>