Add a check for sources.csv

[phanukaev]

The check should reach out to DBLP and verify that all entries in sources.csv generate correct API calls.

[phanukaev]

There is currently a bug in the venue parser:
When two conferences share the same acronym on dblp, one of them will be suffixed with a number to keep them distinct, but the current parser does not allow for this. Cf. for example:

https://dblp.org/db/conf/icalp/index.html

https://dblp.org/db/conf/icalp2/index.html