module system: No shorthand imports syntax in submodules

[roberth]

Describe the bug

When importing into a submodule, imports are not recognized unless the submodule is specified as a function. Also the error message is not helpful.

Normally the syntaxes for modules are interchangable. In particular:

{
    imports = [ ./someFile.nix ];
}

is equivalent to

{...}: {
    imports = [ ./someFile.nix ];
}

This does not work for submodules.

{
    x.y = {...}: { imports = [ ./someFile.nix ]; };
}

should be equivalent to, but doesn't work when specified as

{
    x.y.imports = [ ./someFile.nix ];
}

To Reproduce

This uses Arion as an example and to provide some context as to why one might want this:

$ git clone https://github.com/hercules-ci/arion; cd arion
$ git checkout for-nixpkgs-lib-imports-issue
$ cd examples/minimal
$ NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz ../../run-arion-via-nix cat --show-trace
[...]
The option `services.webserver.imports' defined in `/nix/store/sf3j91p62aznv9iy102sa0838qgjg61n-arion-0.1.0.0-data/share/ghc-8.6.4/x86_64-linux-ghc-8.6.4/arion-compose-0.1.0.0/nix/modules/composition/docker-compose.nix' does not exist.

Expected behavior

The simplification just works.

Metadata
Please run nix run nixpkgs.nix-info -c nix-info -m and paste the result.

NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz nix run nixpkgs.nix-info -c nix-info -m
warning: unknown setting 'sandbox-fallback'
 - system: `"x86_64-linux"`
 - host os: `Linux 5.2.13, NixOS, 19.09beta206.e6b068cd952 (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
warning: unknown setting 'sandbox-fallback'
 - version: `nix-env (Nix) 2.2.2`
warning: unknown setting 'sandbox-fallback'
 - channels(user): `""`
warning: unknown setting 'sandbox-fallback'
 - channels(root): `"nixos-19.09beta206.e6b068cd952, nixos-19.03-19.03.173506.2dfae8e22fd, stable-18.09.2574.a7e559a5504"`
warning: unknown setting 'sandbox-fallback'
 - nixpkgs: `/nix/store/l9k8ycmrdn86a1lzgnx5c4ckld7k2l56-master.tar.gz`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[infinisil]

With #75031 you can turn off this odd behavior for specific submodules by replacing types.submodule with types.fullSubmodule {}, see #75031 (comment)

[stale]

Thank you for your contributions.

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