Hi Guix,

after it was reported that the plain CRAN URI only points to the latest
version of a package and previous versions are archived at a different
URI, I took Dave’s advice and wrote a ‘cran-uri’ procedure that produces
a list of URI strings for a given R package name and a version.

Only one of the two URIs will be correct as the archives don’t contain
the latest version.  The first patch adds the procedure, the second
makes the importer generate a package expression containing the
procedure call, and the last updates all existing R packages to use this
procedure.

I’d like to amend and push Vicente’s useful patches for r-dplyr once
this patch set has been accepted.

Thanks for your patience!

~~ Ricardo