Submodules links are broken when no prefix is defined

[Julien Robert]

When no prefix is supplied, the links it generates for submodules are invalid:

• https://github.com/cosmos/vanity/blob/main/api/index.html#L53-L56
• https://github.com/leighmcculloch/vangen/blob/master/generate_package.go#L34

[Leigh McCulloch]

Thanks for reporting the bug. We should be producing the URL outside the template using the url.JoinPath method.

[Leigh McCulloch]

@julienrbrt I've opened a fix here: #18. Could you try it out and see if that addresses the issue completely?