feat: standalone Home Manager configurations per host

[clo4]

Adds support for Home Manager configurations to Blueprint, both standalone and as a system configuration module.

Related to #36
Closes #35, closes #45

The idea is that Home Manager configurations are defined per-host, as opposed to per-user. This gives users the most flexibility: instead of being tied to a single configuration per username, users can choose to either define a totally unique configuration for a particular host, or use a module to define most of the common configuration and apply light changes on top of it.

The following import paths are supported:

• hosts/{hostname}/users/{username}.nix
• hosts/{hostname}/users/{username}/home-configuration.nix

NixOS or nix-darwin hosts

For hosts containing a system configuration, the Home Manager module and any users defined for that host will be automatically imported, along with setting useGlobalPkgs and useUserPkgs to true (default, can be overridden). This means, outside of very specific use cases, users will not need to change any home-manager options.

Standalone configurations

To support the home-manager CLI, the homeConfigurations attribute is defined per-system under legacyPackages. The packages attribute is intended to be a flat set of derivations, and the top-level homeConfigurations cannot be used since pkgs must already be instantiated (which requires a system) to be used as an argument to home-manager.lib.homeManagerConfiguration. This can be changed in the future if the home-manager CLI becomes deprecated or supports a different output name.

The home.homeDirectory and home.username options will be set automatically, as the username is defined in the path to the file and the homeDirectory can be inferred by the target system. Home Manager set these automatically prior to version 20.09, but this was removed to make evaluation pure.

This PR also allows hosts to not define a system configuration if the directory contains a users subdirectory.

[zimbatm]

thanks for the contrib, it seems to be going in the right direction overall

[clo4]

Changed some stuff.

• Users are now defined under the users subdirectory of a host.
  • TODO: don't fail if a home-configuration.nix doesn't exist, since people may be using this directory already as it's fairly logical
• home-manager is automatically imported for a given host if it's an input and the host has any number of users defined
• Standalone HM configurations automatically set the username
• Commit has a description

[clo4]

Ready for review.

• Automatically configuring home.homeDirectory was moved to its own commit because I wasn't certain about it, but it in practice feels good to not configure it since we already know the operating system and the username.
• I put the docs in what I think is the most logical place, but please let me know if you'd rather shuffle it around somewhere else.
• I'm extremely open to nitpicks! :p

[zimbatm]

Thanks, this is great.

Last round of small feedback and then this is ready to be merged.

[clo4]

Finally got power back! Had an outage that lasted a few days, then had quite a backlog of stuff to do outside of work. Ready for review round 2

PR description has also been fully updated with the current overview.

[zimbatm]

ok this looks good-enough to be merged.

Tests are missing, but we don't have any tests so it's unfair to ask for some.

In the future we might also want to address a top-level users/ folder that includes UID mapping. But that can be added later and will benefit from the practical experience of having that merged in.

[zimbatm]

thanks @clo4 !

[clo4]

I did create some files to test these changes locally and I've been using it for my own configuration throughout the process, so I'm fully confident that it works as expected. I've never dealt with writing proper tests in Nix, so depending on how much time and energy I have over the coming days I might toy around with that. If anything good comes out of it I'll PR it too :)

[zimbatm]

Looking forward to it, testing is one of my main weakspots as a software engineer 😅

Maybe we could just have an ./examples folder with a bunch of sub-flakes in there. It could double as an integration test and a way to demonstrate the various ways to use blueprint.