Warn on load when downstream dep is incompatible

[lionel-]

A user mentioned a forward compatibility issue between rlang 0.4.0 and dplyr 0.7.8: tidyverse/dplyr#4600.

As I'm about to release rlang 0.4.1, I'd like to experiment with a load-time warning to detect packages known to be forward-incompatible. Loading rlang with an old dplyr would give this warning:

Warning message:
As of rlang 0.4.0, dplyr must be at least version 0.8.0.
✖ dplyr 0.7.8 is too old for rlang 0.4.1.
ℹ Please update dplyr with `install.packages("dplyr")`.

On Windows platforms, we link to rstats-wtf/what-they-forgot#62 where the second comment provide a rough guide on proper updating of packages.

Warning message:
As of rlang 0.4.0, dplyr must be at least version 0.8.0.
✖ dplyr 0.7.8 is too old for rlang 0.4.1.
ℹ Please update dplyr to the latest version.
ℹ Updating packages on Windows requires precautions:
  <https://github.com/jennybc/what-they-forgot/issues/62>.

[hadley]

Doesn't purrr need this treatment too?