rstudio: Fix recompilation issue with custom package set

This attempts to fix the issue described at
https://github.com/NixOS/nixpkgs/pull/22219#issuecomment-291801133.
Any change to the custom packages passed to RStudio causes this to
completely rebuild RStudio, which is completely unnecessary and also a
bit of a hindrance as it's a fairly slow build.

This rolls back most of that old PR, and instead implements something
more like rWrapper.  Existing configurations with the old useRPackages
will break.