Check the source directory for code

[sdassow]

This addresses the confusing output mentioned in: #44

Making sure the source directory contains code seems like an easy way to prevent this from being confusing.

[Jacalz]

I wonder if it would be better to just call out to go list in cases like this?

[andydotxyz]

For me it makes sense to use the API/stdlib over calling out to executables to tell us the presence of files.

Unless there is a reason that this could fail when go list would succeed @Jacalz ?

[Jacalz]

The standardised tooling might have some benefits because we don't need to maintain our own solution but yeah, I realise now that it is better to just use the stdlib for this case. I just have a comment inline to make sure that we terminate early to avoid walking the whole file path.