Rename all non-development requirements files to `requirements.txt`

[brettcannon]

This allows for easier automated scanning of licenses on 3rd-party packages. This should let us clean out much of cgmanifest.json.

[brettcannon]

I think jedils_requirements.txt might be the only file that needs renaming (we can stick it in the directory where we have pip install the files and name it requirements.txt there). As such, I'm assigning this to @karthiknadig .