Is your feature request related to a problem? Please describe.
When you developing a flake that will be used by other flakes your flake might come with more inputs than the end-user needs.

Several inputs might only be needed during development/testing. Examples:

• Even in this repo there is a flake input for nixpkgs regression test:
  https://github.com/NixOS/nix/blob/master/flake.nix#L5
• If I create a library function or an overlay I might want to test towards multiple nixpkgs releases.
• I might be using something like devshell for managing my development dependencies.
• I might use flake-compat and use the flake to pin down the source.

Describe the solution you'd like
I'm not sure how to verify that those inputs you use will only be used for devShell, checks or hydraJobs. But I guess when opting out from inputs you would need a way to determine what inputs needs to be pulled in.

Describe alternatives you've considered
Creating multiple standalone flakes, e.g. one flake in a folder named devshell, another flake in a folder named tests. But unfortunately that makes the main flake not supporting the nix develop and nix flake check commands.