There is a lot of refactoring here too.

There was a check to ensure that if there are components that we plan to install (or update) that are missing from the manifest, then we error out. However, this only worked for components that had an entry in the manifest under the Rust package, but were missing components of their own. For tools like the RLS, when they are missing, they are completely missing from the manifest. They therefore didn't get picked up by the check. This patch adds an extra Vec to track such components and then warns about them.

This adds a new test and removes test `update_removes_components_that_dont_exist`

Historically, this test was added before the check that aborts update on missing
components (which is tested by the new test in the previous commit, as well as
elsewhere), so I think this test was only testing a bug.

Note that the new behaviour is the same as the ancient behviour when the user
uses `--force`