Print a list of all not-found required and optional dependencies

It's very annoying to run `meson` and work your way through a list of required and not-found dependencies one-by-one. Package managers are slow and have a large overhead per-invocation and ideally you want to get a full list, set it to install, and get back to it later.

There is no recourse w.r.t. this when using Autotools, but we can definitely do better. I think it would also be a major plus-point when talking about Meson to other people.

The issue names no files, tests, or entry points beyond running meson, so begin by locating where Meson checks required and optional dependencies and how it reports not-found results. Done means a single configuration run reports the complete list of missing required and optional dependencies for later batch installation.

Written by the indexing model from the issue text.