Add a way to ignore dependencies not available in some systems

[MLopez-Ibanez]

If a package has Rmpi in Suggests it will fail to install with pak, because Rmpi is not built for MacOS. See this example: log.txt

There does not seem to be anyway to avoid this error except ignoring the whole package with Rmpi=?ignore. It would be great if pak could have something like Rmpi=?ignore-os=macos.

Or simply check if the package is not available for some OS and skip it with a warning.