Merge pull request #328272 from TomaSajt/r-packages-migrate-to-json

```bash

nix-shell generate-shell.nix

Rscript generate-r-packages.R cran  > cran-packages.nix.new

mv cran-packages.nix.new cran-packages.nix

Rscript generate-r-packages.R cran  > cran-packages.json.new

mv cran-packages.json.new cran-packages.json

Rscript generate-r-packages.R bioc  > bioc-packages.nix.new

mv bioc-packages.nix.new bioc-packages.nix

Rscript generate-r-packages.R bioc  > bioc-packages.json.new

mv bioc-packages.json.new bioc-packages.json

Rscript generate-r-packages.R bioc-annotation > bioc-annotation-packages.nix.new

mv bioc-annotation-packages.nix.new bioc-annotation-packages.nix

Rscript generate-r-packages.R bioc-annotation > bioc-annotation-packages.json.new

mv bioc-annotation-packages.json.new bioc-annotation-packages.json

Rscript generate-r-packages.R bioc-experiment > bioc-experiment-packages.nix.new

mv bioc-experiment-packages.nix.new bioc-experiment-packages.nix

Rscript generate-r-packages.R bioc-experiment > bioc-experiment-packages.json.new

mv bioc-experiment-packages.json.new bioc-experiment-packages.json

```

`generate-r-packages.R <repo>` reads  `<repo>-packages.nix`, therefore

`generate-r-packages.R <repo>` reads  `<repo>-packages.json`, therefore

the renaming.

The contents of a generated `*-packages.json` file will be used to

create a package derivation for each R package listed in the file.

Some packages require overrides to specify external dependencies or other

patches and special requirements. These overrides are specified in the

`pkgs/development/r-modules/default.nix` file. As the `*-packages.nix`

`pkgs/development/r-modules/default.nix` file. As the `*-packages.json`

contents are automatically generated it should not be edited and broken

builds should be addressed using overrides.