CRAN loader: set revision dates

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

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Migrated from T2533 (view on Phabricator)