Allow for skipping unavailable suggests

[jeroen]

I would like to use pak for r-universe CI/CD and reverse dependency checks. A problem is that pak(dependencies = TRUE) errors if an optional dependency is unavailable.

Sometimes a package has a Suggests on another package which is unavailable. Example from CRAN include that a suggested package has been archived, or is only available for a specific platforms, or was never on CRAN in the first place. However the main package should still pass checks (otherwise it would not be on CRAN).

If possible, it would be great to have an option to let pak give a warning instead of an error (cf install.packages) when an optional dependency is unavailable.