modules: fix evaluation when modules emit warnings

[sandydoo]

Description

The warnings option is used by functions like lib.mkRenamedOptionModule and lib.mkChangedOptionModule to warn the user to update their configuration. Without this option, evaluation fails on these functions with a missing attribute error.

Repro

You can reproduce the error by using a renamed option, like postgres.enable (instead of services.postgres.enable).

{ ... }:

{
  postgres.enable = true;
}

Before

Building shell ...
warning: applying 'toString' to path '/home/sandydoo/code/cachix/devenv/src/modules/services/postgres.nix' and then accessing it is deprecated, at «stdin»:0
error:
       at /home/sandydoo/code/nixpkgs/lib/modules.nix:326:9:

          325|         options = checked options;
          326|         config = checked (removeAttrs config [ "_module" ]);
             |         ^
          327|         _module = checked (config._module);

       … while evaluating the attribute 'config'

       error: The option `warnings' does not exist. Definition values:
       - In `/home/sandydoo/code/cachix/devenv/src/modules/services/postgres.nix': [ ]

After

Building shell ...
warning: applying 'toString' to path '/home/sandydoo/code/cachix/devenv/examples/postgres/devenv.nix' and then accessing it is deprecated, at «stdin»:0
trace: warning: The option `postgres.enable' defined in `/home/sandydoo/code/cachix/devenv/examples/postgres/devenv.nix' has been renamed to `services.postgres.enable'.

19:21:08 system     | postgres.1 started (pid=631304)

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View logs

[sandydoo]

warning: applying 'toString' to path '/home/sandydoo/code/cachix/devenv/examples/postgres/devenv.nix' and then accessing it is deprecated, at «stdin»:0
This extra warning looks like a bug in nixpkgs. It's converting the file path to a string to display it in the next warning, but clearly the toString machinery is throwing spurious warnings in this case.