Custom repo URIs

It's up to you. What URL do you want to see in the address bar? (Obviously it's clear in the example you gave, but just for the purposes of explanation.) The URIAlias redirects to the repository's primary URI. So if you wanted https://sourcegraph.com/sourcegraph/srclib to redirect to https://sourcegraph.com/github.com/sourcegraph/srclib, then the primary URI would be the github URI and the alias would be the sourcegraph URI. But in this case we want the opposite behavior.