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.

pkgs/development/r-modules/README.md

@@ -55,28 +55,23 @@ available.
 
 ## RStudio
 
-RStudio by default will not use the libraries installed like above.
-You must override its R version with your custom R environment, and
-set `useRPackages` to `true`, like below:
+RStudio uses a standard set of packages and ignores any custom R
+environments or installed packages you may have.  To create a custom
+environment, see `rstudioWrapper`, which functions similarly to
+`rWrapper`:
 
 ```nix
 {
     packageOverrides = super: let self = super.pkgs; in
     {
 
-        rEnv = super.rWrapper.override {
+        rstudioEnv = super.rstudioWrapper.override {
             packages = with self.rPackages; [
-                devtools
+                dplyr
                 ggplot2
                 reshape2
-                yaml
-                optparse
                 ];
         };
-        rstudioEnv = super.rstudio.override {
-            R = rEnv;
-            useRPackages = true;
-        };
     };
 }
 ```