Implement update_packages

[hadley]

Following proposal in #975

This should be implemented as a new generic update_remote() function for remote objects, and will probably also need some code to create the remote object from a DESCRIPTION. This should make it possible to share a lot of code with the existing installation functions.

[lock]

This old issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with reprex) and link to this issue. https://reprex.tidyverse.org/