[Rd] copy-before-ignore in R CMD build

[Ben Bolker]

  As currently implemented, R CMD build copies everything in the package 
directory to a working directory, then unlinks those in .Rbuildignore 
(see this comment by Jim Hester from 2018).

https://github.com/r-lib/rcmdcheck/issues/90#issuecomment-435938362

   If one accidentally happens :-( to create a directory with many 
gigabytes of junk in it, say from reverse-dependency-checking a widely 
used package, then the presence of that directory (even if 
.Rbuildignore'd) will make R CMD build hang as it tries to copy the 
contents of the directory ...

   I realize it's considerably more complicated to skip .Rbuildignore'd 
files in the process of copying (I don't see offhand how to do it 
without some fairly fancy unix-`find`-like tool, as opposed to a 
straight-up `file.copy(..., recursive = TRUE)`, but I wonder if there's 
any appetite for changing this behaviour?

   cheers
    Ben Bolker