Removed already-installed packages from install command

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: fixes Don't request installing packages already present in package.json #416
• That issue was marked as status: accepting prs

Overview

Separates logMissingPackages out from the multi-purpose reportConversionResults and directly calls it in convertConfig. It'll now take in the result of reading --package (normally package.json) and remove any package names contained in dependencies or devDependencies from the list of suggested packages to install.

Per the notes in #416, we can't look at the extends value of an ESLint configuration file for packages that need to be installed - even though eslint-config-prettier isn't added to plugins. This PR adds in a special case for eslint-config-prettier existing in an extends. I don't know of any other ones we'll need to add to that special case... for now.

Also adds plugin: before plugin names, because that seems to be recommended by the ESLint docs.

[JoshuaKGoldberg]

I don't remember why this was added before, but it isn't doing anything... 🤷