[Feature request] Dry run or verbose option to the `install_*` functions

[dmurdoch]

I made some changes to my Github R CMD check action for rgl, and suddenly it was trying to install a couple of thousand packages instead of a couple of hundred, and runs were taking a really long time. To debug what I did wrong, it would be nice to have a "dry-run" option that says what the install functions plan to do (and why) without doing it. For example,

install_deps("YYYY", dependencies = "soft", dry_run = TRUE)

might print something like:

Dependencies to install for YYYY:
Package XXXX is in Depends for package YYYY.
Package ZZZZ is in Imports for XXXX.
Package WWWW is in Imports for YYYY.

etc., listing one line for each package that would be installed. Alternatively to a dry_run option would be a verbose option that would print lines like the ones above just before actually doing each install, so I could tell why package ZZZZ was being installed, for example.