Don't request installing packages already present in package.json

[JoshuaKGoldberg]

🚀 Feature Request

Right now, tslint-to-eslint-config will report any packages that must be required for the generated ESLint configuration:

We already read from package.json and support a --package flag. We could save a little bit of CLI output space skipping printing packages that are already listed in package.json as a dependency or devDependency.

Existing Behavior

⚡ 7 packages are required for this ESLint configuration. ⚡
  npm install @typescript-eslint/eslint-plugin @typescript-eslint/parser @typescript-eslint/eslint-plugin-tslint eslint eslint-plugin-import eslint-plugin-jsdoc eslint-plugin-prefer-arrow --save-dev

Change Proposal

If all those packages already exist, the ⚡ part of CLI output could be skipped.

If only some already exist, we could skip printing them.

⚡ 3 new packages are required for this ESLint configuration. ⚡
  npm install @typescript-eslint/eslint-plugin-tslint eslint-plugin-jsdoc eslint-plugin-prefer-arrow --save-dev

[JoshuaKGoldberg]

A little bit of notetaking. Some extends values are, in fact, packages that should be installed. Others are not.

    "extends": [
        "plugin:@typescript-eslint/recommended",
        "plugin:@typescript-eslint/recommended-requiring-type-checking",
        "plugin:eslint-config-prettier",
        "plugin:eslint-config-prettier/@typescript-eslint"
    ],

It's tempting to add extends to the list of packages to install... but that's not reasonably doable at this time.