[PATCH] Add recursive CRAN importer

[Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>]

attached is a patch series that gives us a recursive CRAN importer.  I still
haven't documented it in the manual, because I'm not sure if the
implementation is acceptable.

Simply put, the procedure that produces a package expression now returns
multiple values.  The first is the package expression, the second is a list of
dependencies (with their upstream names).  This allows us to go through that
list and check if any unpackaged inputs are among them.

The recursive importer stops when a package cannot be imported (e.g. when a
bioconductor package has unpackaged CRAN dependencies), or when all packages
either already existed at runtime or have been packaged.

To make this work well I had to create a list of packages that should not be
imported (because they are part of the default R installation), and I had to
make sure that R packages always get a predictable Guix package name.

Comments are very welcome!

~~ Ricardo