add support for 3rd party index

[mangas]

Implements #134

• Add flag to override the index to fetch the tarball link from.

[mangas]

The cosmetic changes were done by zig fmt on save, if needed I can remove them.

[marler8997]

thanks for the contribution. I went ahead and ran zig fmt and pushed to master. Do you mind rebasing and/or reworking the change off of master so I don't have to wade through the formatting changes?

[mangas]

There are still some small cosmetic changes but seems a lot easier to parse now. Let me know if this is close enough otherwise I can try to make the changes and turn off the formatting

[marler8997]

such a simple change/enhancement, I'm just going to merge this as is. Would be nice to add a test but can be done later.

[mangas]

I'll have a look at the tests and see if I can add one that uses this flag