SCP24: Do not require packages without dependencies

[Gallaecio]

We can make SCP24 more flexible by only requiring packages that actually bring (new) dependencies in that could cause version conflicts.

For example, scrapy-deltafetch only depends on Scrapy>=1.1.0, so we might as well not trigger SCP24 if it is missing, since dependencies in your requirements.txt file should never cause issues with scrapy-deltafetch.