Regular CRAN checks for tidyverse deprecations

[hadley]

With the latest changes to lifecycle and testthat, you should only ever discover soft deprecations when actively developing your package (and most regular deprecations won't cause a CRAN failure). But there are lots of packages that are not actively developed, and it would be nice to have some way to gently notify those maintainers that they're using functions that we no longer recommend.

I was thinking that every 6 months, we could run all tests and examples for all packages that depend on a tidyverse package, collect and summarise all the deprecations, and email the maintainer. It would be a purely informative email since no action would be required, but it means that folks will here from us 2-4 times before we move from a soft deprecation to a regular deprecation, or a regular deprecation to defunct.