repometadata: FromLibrary should assign Transport

[jskeet]

When repometadata.FromLibrary was first created, we didn't have a standard way of determining the transport. We now do, so the common function should do it. We can then remove assignments to RepoMetadata.Transport from per-language code.

[perashanid]

Hi! I'd like to work on this issue. I'll move the Transport assignment logic to repometadata.FromLibrary and remove it from per-language code.

[JoeWang1127]

Reopen this issue since the fix caused an regression in Rust.