Update make validate so that it ensures that definition is present in index.yaml

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The make validate target should raise an error if a definition is not present in index.yaml. We can do this by loading the index config file and then doing a simple check based on the input symbol (i.e. :hk should be present in index.yaml.

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Additional checks:

We currently allow incorrect naming of the files in our index.yaml file. We should require the following:

1. We should have a matching file in the repo, meaning if we put 'us.yaml' then that file must exist.
2. We should enforce '.yaml'. Right now we will allow anything.