Platform-specific package managers

[shadows-withal]

Currently, all of our package managers are global. This means that they'll be shown for every arch/target. This is fine for a lot of package managers, such as npm or Cargo, but there are also lots of platform-specific ones, including:

• Homebew, for macOS
• apt/yum/dnf/zypper/pacman/you name it for Linux
• scoop for Windows

...and way more. Package managers should therefore be able to have a platform scope attached to them, only showing up on the platforms they actually support.

To-Do

• Etch out configuration design (@ashleygwilliams made an interesting point - package manager declaration is effectively data, not config, maybe it should live in a different file/space?)
• Handle Homebrew correctly - cargo-dist returns a [name].rb file that we need to properly rename
• Implement a general version