Join URLs properly.

[vitorhnn]

Currently, --repo-url has to be passed to sync with a trailing slash, otherwise we pass invalid URLs over to ureq:

called `Result::unwrap()` on an `Err` value: Http { url: "https://swifty.peanutcommunityarma.com@tc_mirrorform/mod.srf", source: Transport(Transport { kind: Dns, message: Some("resolve dns name 'tc_mirrorform:443'"), url: Some(Url { scheme: "https", cannot_be_a_base: false, username: "swifty.peanutcommunityarma.com", password: None, host: Some(Domain("tc_mirrorform")), port: None, path: "/mod.srf", query: None, fragment: None }), source: Some(Custom { kind: Uncategorized, error: "failed to lookup address information: Name or service not known" }) }) }

This is due to us joining URLs through format!. We should switch to the url crate.