Before, devShell could be set to a submodule config, a package def, or a
function taking module args and returning a submodule config. This
changes the last form to take the package set instead.
This enables cleaner configuration by not needing each option to
individually be a function of pkgs. Passing pkgs also gives more
flexibility as all the module args are available under the `moduleArgs`
attr.
Code that relied on module args not directly in the package set will now
have to access them from the `moduleArgs` attr.
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