• part of All command line arguments that alter output should be configurable in config file #47

Prior to this change, if you ran the following:

$ create-api generate --config wrong/path/to/config.yml schema.json

The generator would ignore the fact that wrong/path/to/config.yml was an invalid path as long as it had the right extension. Instead, it would just revert to using the default config, which might leave the user confused as to why their options aren't being applied.

It most likely did this because the default value was always set to .create-api.yaml and we don't necessarily expect this file to always exist, and we wouldn't want to error if it didn't since the user didn't explicitly define it.

It was a bit hard to check this in the Generate command with the config alone since it's just a String and Swift Arguments Parser doesn't give you a way to differentiate between default values and user defined values out of the box.

For example, if you specified --config .create-api.yaml (the default value) explicitly, we'd still want to error if that file didn't exist.

To fix this, I opted to introduce a new ConfigFileLocation type that conforms to ExpressibleByArgument. By doing this, I am able to differentiate between the default value, and a user defiled value passed into init?(argument:). I then added an extension to expose the resolved fileURL which will throw an error if the file doesn't exist like we expect.

I also added some test coverage to make sure errors are thrown (not something we typically do)