Merge legacyPackages and packages in perSystem

[brianmcgee]

Is your feature request related to a problem? Please describe.

When using https://github.com/numtide/devshell I cannot use perSystem.devshell.mkShell because of the following:

blueprint/lib/default.nix

Lines 23 to 25 in 4f88aed

	perSystem = lib.mapAttrs (
	_: flake: (flake.packages or flake.legacyPackages or { }).${system} or { }
	) inputs;

This prefers the packages attribute over legacyPackages, and in the case of devshell it exposes different things between those two attribute sets.

Describe the solution you'd like

I'm not sure what the correct solution looks like. Maybe a recursive merge, giving precedence to packages?

Describe alternatives you've considered

For now I'm using inputs.devshell.legacyPackages.${system}.mkShell.

[zimbatm]

Same, no immediate good answer comes to mind. One it would be good to move away from legacyPackages, but then we need an answer to per-system lib constructors like devshell.mkShell.

[zimbatm]

fixed in #17