auto-detect cruft

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• when you call a library, but don't use it
• when you have data files that you don't load
• when you have script files that you don't execute

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• packages: currently we use requirements to scan the code and record the names of the packages that are called. If they use one of the functions we have shimmed, then we'll know it, and we can compare the two lists. But we aren't to shim every function. So how will we know if they do or don't use a function in a package?
• input files: compare the list of files in the directory to the list of files called by our shimmed functions. Report any differences.
• output files: ditto

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https://github.com/dreamRs/prefixer

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See also #29