sysreqs might not be available for new version in extra CRAN metadata

[gaborcsardi]

If the package was just published, e.g. r-hub/actions#24

I just set an hourly update, so this should be rare, it should mostly happen for R-devel, anyway, because R-HUB binaries have the correct metadata and by the time PPM builds the binary the extra metadata is updated.

TODO:

• we should probably try look up the metadata, if we can, from the DESCRIPTION on CRAN. (This would fail on PPM, though, but that's ok, it is not needed on PPM, see above.)
• print that the sysreqs data is unknown, when the install plan is printed, if we failed to look it up.

We could also check the sysreqs right when the package is being installed, but that should not be necessary, given the above changes.