Codelists: Handle extensions patching others’ codelists

[jpmckinney]

Running find . -name '+*' and find . -name '-*', I can't find an extension that does this, but if/when one does, the code probably doesn't handle it. (I wrote this note sometime in 2018 or 2019, so I assume the code doesn't handle it.)

[jpmckinney]

_codelist_url has an exception for ocds_statistics_extension, which patches statistic.csv from bids.