Handle package cellars

[ErdaradunGaztea]

{renv} allows specifying local package "repositories" in the form of "package cellar": https://rstudio.github.io/renv/articles/cellar.html. Since there is a structure to it and I've seen a use case for it already, it makes sense to add another batch of functions for that. It may be difficult to extract DESCRIPTION files for dependencies, though.