home-manager: move profile management

[rycee]

Description

This commit separates profile management (setting profile and creating GC root) from file management (removing and adding managed files within the user's home directory).

This is a step towards deprecating profile management within the activation script, instead relying on the caller of the activation script managing the profile.

Checklist

• Change is backwards compatible. Almost entirely, hypothetically somebody may rely on the profile switch happening in the link files activation script block but

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

[rycee]

@blaggacao Hi! Since you did the original support of "nix profile", would you mind trying out this PR? It is related to some simplifications I'm working on. In this case the home-manager profile update is moved and simplified. Note in the simplificated code I always use nix-env for the profile management.

In my tests this has worked well even when the user uses the modern "nix profile" style profiles. Still would be good with an independent test, though 🙂

[blaggacao]

Hi @rycee, I'm taking a break and keeping myself away from Nix setups these days, at least for some time.

Therefore, I hope my validation can be skipped.

I will maintain the intention to fix anything related to this should I stumble over it: I have a starting point, now, thank you for pinging me!

PS: The LoC net balance of this PR seems definitely worthwhile and promises greater long term stability.

_{PPS: Happy new year!}

[rycee]

@blaggacao No worries, have a nice vacation away from Nix! 😀