non-trivial flakes

[Sk7Str1p3]

Is your feature request related to a problem?

Currently flake must be single file without any imports, concatenation, etc. Some flake are enormous and reading them as single file is really difficult.
We already have flake-parts but they are not embedded in nix and have some limitations like disability of importing inputs from many files

Proposed solution

Importing inputs from other .nix files;
Importing outputs from other .nix files (flake-parts)
Allow custom outputs (flakeModules)

Additional context

none

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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[Sk7Str1p3]

Dup of #5663, closing