nixos-container: fix `nixpkgs` container options being ignored

Since the introduction of option `containers.<name>.pkgs`, the
`nixpkgs.*` options (including `nixpkgs.pkgs`, `nixpkgs.config`, ...) were always
ignored in container configs, which broke existing containers.

This was due to `containers.<name>.pkgs` having two separate effects:
(1) It sets the source for the modules that are used to evaluate the container.
(2) It sets the `pkgs` arg (`_module.args.pkgs`) that is used inside the container
    modules.
    This happens even when the default value of `containers.<name>.pkgs` is unchanged, in which
    case the container `pkgs` arg is set to the pkgs of the host system.
    Previously, the `pkgs` arg was determined by the `containers.<name>.config.nixpkgs.*` options.

This commit reverts the breaking change (2) while adding a backwards-compatible way to achieve (1).
It removes option `pkgs` and adds option `nixpkgs` which implements (1).
Existing users of `pkgs` are informed by an error message to use option
`nixpkgs` or to achieve only (2) by setting option `containers.<name>.config.nixpkgs.pkgs`.

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