Improve version requirements in is_installed() and check_installed()

[lionel-]

Branched from #1331.

• DESCRIPTION-style requirements are now supported, e.g. is_installed("rlang (>= 1.0.0)".
• New compare argument to supply requirement info programmatically.
• When < or <= is used, check_installed() doesn't prompt for install.

This will allow support for check_installed() in pkgload, so that we get install prompts on missing deps.

[lionel-]

@hadley In pkgload, a failing version requirement only issues a warning instead of an error, this way the package can be loaded anyway. I just want to check this is still the behaviour we want to support since that might be slightly tricky to implement. (Probably we need to signal a classed condition before calling invokeRestart("abort").)

[hadley]

In pkgload, I think we can just protect with some tryCatch()s, like we need to do for namespace imports.

[hadley]

Oh but the idea is to now use check_installed() directly in pkgload, so we'd be replacing the warning with a prompt to install the package; I think that's ok.

[lionel-]

Oh but the idea is to now use check_installed() directly in pkgload, so we'd be replacing the warning with a prompt to install the package; I think that's ok.

ok cool, that will make things simpler!

We now take an optional action callback that is called when the user chooses to update/install dependencies.