Sanity checks of the source data

### Context
We receive a file in tsv format (see #10) which contains one or more rows of data about projects NLnet has funded. Some of the URLs intended to map to codebases do not actually map to code repositories e.g. they may map to the owner of various repos on github.com. #11 discusses what we could do when we find these instances.

Therefore it's worth sanity checking the URLs instead of blindly making requests to the online repo hosting services and catching & reporting problems early. This may then enable NLnet to revise the source data so it has links to the appropriate repositories.

Start by tracing how the TSV source data is ingested and where URLs are sent to online repository hosting services. The work is done when URLs that do not identify code repositories are detected and reported before repository requests are made, allowing the source data to be revised.

Written by the indexing model from the issue text.