Possible to create a shared user package cache?

[colearendt]

Along the lines of what is mentioned in #1 , is it possible to share a package cache across users?

For documentation, this was requested in Packrat:

rstudio/packrat#357

[kevinushey]

This is possible by setting the RENV_PATHS_CACHE environment variable, but we should probably think about APIs for getting / setting this more comfortably.

[kevinushey]

This is implemented now and I think RENV_PATHS_CACHE is the right way to go (so users can control it in e.g. project-local .Renviron or similar). We can revisit if we discover a specific deficiency with this schema.