[r-packages] allow whitespace in packages

[dpprdan]

This is a small change to allow for whitespace in the packages specification, so that using

"packages": "cli, rlang"

does not lead to an error.

[eitsupi]

Thanks for this!
Could you add a new test case and bump the minor version?

[dpprdan]

@eitsupi Is the test correct like this? I am not sure.

I'll add a similar test for the apt-packages feature. It would only be consistent if it also allowed for whitespace in the package list. It already does IIUC, but a test might be useful to prevent a regression? Feel free to revert, if this doesn't make sense.

[eitsupi]

Thanks!