Hard-detect common arch-specific package managers

[manyinsects]

When a user adds a custom package manager that we know is only available for a specific target, we should (until #604 is fully implemented) try to guess this.

• Homebrew, if you don't have the cargo-dist integration
• scoop, for Windows
• Linuxbrew? Not sure what would be different about this
• Various Linux-only package managers (apt, yum, zypper, pacman, rpm, pkgsrc, etc)