refactor: move package managers into a dir

[rigor789]

The goal is to clean up package management throughout the CLI - this is the first step (and it's WIP) towards that goal.

Right now we have 2 services responsible for package management:

• PackageManager
• PackageInstallationManager

The goal is to combine them with a decoupled API from the specifics of each package manager. The interfaces have been created for NPM, and are tailored to npm's cli flags and terminology. We should abstract the specifics like save-dev should just be a combination of dev: true and save: true and each package manager would then use the correct flags: --save-dev or -D etc.