Add back go-source tag

[abhinav]

pkg.go.dev uses the go-source for VCS hosts that it doesn't recognize.
We should add back go-source.

Thanks to @hacdias for reporting:

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Correct me if I'm wrong, but this seems like a regression. pkg.go.dev still uses go-source for unknown VCS systems: https://pkg.go.dev/about#source-links - right now, Sally only supports GitHub, but it'd be great if it were more generic.

Originally posted by @hacdias in #83 (comment)

[SoulPancake]

Can I do this again haha @abhinav
I was the one who removed it

[abhinav]

Sure, @SoulPancake, go for it!

[hacdias]

@SoulPancake if you're at it, it would be great if you could take this PR into account (#128) and added some field that lets users override the source value via the config. If not, I can always make another PR if that'd be welcomed.

[abhinav]

It seems to me that if we want to support alternative VCSs per #128, we can't hard-code it to GitHub's directory/file path scheme like we did previously. We'll need something like a fileTemplate and a dirTemplate field to allow setting them to arbitrary values per https://github.com/golang/gddo/wiki/Source-Code-Links. Does that sound right to you, @hacdias?

[hacdias]

Yeah, I think so. I would add both variables you mentioned. Only if both are set, add go-source.

By default, I wouldn't set go-source, as chances are most people will be using known VCS services/systems.