[RRFC] `npm explain` should work even absent a lockfile and node_modules

Motivation ("The Why")

For example, https://github.com/npm/cli/issues/4311#issuecomment-1030707030 - when the first (ever, or first locally on a project without a lockfile) install fails, there's no lockfile and no node_modules, and no way to know why something's attempting to be installed.

Arborist has the ability to load the ideal tree in this circumstance, so there's no technical barrier I'm aware of that prevents this command from working.

Example

Solely from a package.json, npm explain foo would work without requiring a lockfile, or reified dependencies.

How

Current Behaviour

When I run npm explain foo without a lockfile or node_modules present, i get npm ERR! No dependencies found matching foo.

Desired Behaviour

I'd expect to get the same result as when either are present - the actual explanation of where foo comes from.

(If this seems like something simple enough to not require an RFC, i'd be happy to make a PR to add it)

Start with the npm explain behavior described in the issue and Arborist's ability to load the ideal tree. Reproduce the command using only package.json, without a lockfile or node_modules, and compare it with runs where dependencies are present. Done means npm explain foo reports where foo comes from in the lockfile-free case.