[Rd] Errors when base package version is specified

[Duncan Murdoch]

On StackOverflow (https://stackoverflow.com/q/63409565/2554330), a user 
was confused by the error messages from R when the DESCRIPTION file for 
his package specified

   Imports:  stats >= 4.0.2

The message from an attempted source build in R 3.6.1 said:

   Warning in install.packages :
     dependency ‘stats’ is not available
   installing the source package ‘doebioresearch’

and later

   Error in loadNamespace(j <- i[[1L]], c(lib.loc, .libPaths()), 
versionCheck = vI[[j]]) :
     namespace 'stats' 3.6.1 is already loaded, but >= 4.0.2 is required
   Calls: <Anonymous> ... namespaceImportFrom -> asNamespace -> 
loadNamespace
   Execution halted

Would it be worth giving an error or warning right away based on the 
Imports line?  It doesn't make sense to specify the version of a base 
package.  If he had specified R >= 4.0.2 I think the messages for R 
3.6.1 users would have been less confusing.

Duncan Murdoch