Split nupac.nu into more specific entities

[skelly37]

Describe the issue

Desired output:

• main nupac.nu file with the currently exported commands, basically our public interface
• file with package handling logic: installation and their maintenance
• file with metadata parsing logic
• file with simple helpers & constants & "constants"

Argumentation

• we won't block ourselves by working on a single file
• we will waste less tiime on merging
• the code will be more re-usable
• it will be easier to find something in the codebase

Screenshots and other helpful media

No response

Additional context

No response