Warn if NAMESPACE incompatible with Imports/Suggests

[krlmlr]

We know which packages are imported from via @import and @importFrom. If we don't import but use :: in the code, it's a "Suggests". Perhaps add a new tag @suggest to make it explicit (and avoid parsing the code)?

We now have the necessary infrastructure (in particular {desc}) so that this should be fairly easy to add?

Opt-in, perhaps run by default and give a message if imports/suggestions are different? Should this go to https://github.com/gaborcsardi/roxygenlabs first?

Gains:

• Saves frustration when problems only come up with R CMD check.
• Allows to document the rationale behind each imported/suggested package. AFAIK DESCRIPTION doesn't support comments (only *Note fields).

[hadley]

I wouldn't like to add these automatically, but I think it would be reasonable to emit a warning saying that packages need to be added to imports/suggests.

[hadley]

Six years later and we haven't made any progress towards this so I think it's save to say it's unlikely to ever happen.