Currently, the CRAN loader always sets revision dates to None.
It makes sense for the date to be optional, as it may be missing; but we should use the `Date/Publication` and/or `Packaged` fields when they are available (and possibly others).
Motivation:
* "time-less revisions also break the provenance model as you wouldn't know if a time-less revision comes before or after one with a time" -- @zack
* the Debian loader already does it