What does this PR try to resolve?

It's very common for users to attempt to use the pseudo-URLs that GitHub or other providers provide in the form
git@github.com:rust-lang/rust.git as a source in Cargo.toml, which are the default format accepted by OpenSSH. Unfortunately, these are not actually URLs, and unsurprisingly, the url crate doesn't accept them.

However, our error message is unhelpful and looks like this:

invalid url `git@github.com:rust-lang/rust.git`: relative URL without a base

This is technically true, but we can do better. The user actually wants to write a real SSH URL, so if the non-URL starts with git@, let's rewrite it into a real URL for them to help them and include that in the error message.

git@ is the prefix used by all major forges, as well as the default configuration for do-it-yourself implementations like Gitolite. While other options are possible, they are much less common, and this is an extremely easy and cheap heuristic that does not necessitate complicated parsing, but which we can change in the future should that be necessary.

This also avoids the problem where users try to turn the pseudo-URL into a real URL by just prepending ssh://, which causes an error message about the invalid port number due to the colon which they have not changed. Since they can just copy and paste the proposed answer, they'll be less likely to attempt this invalid approach.

Fixes #13549

How should we test and review this PR?

1. cargo init pkg1
2. cd pkg1
3. Edit Cargo.toml to add the dependency line bytes = { git = "git@github.com:tokio-rs/bytes.git", tag = "v1.10.0" }.
4. Run cargo build
5. Notice that the error suggests a URL to try.
6. Replace the Git URL with the suggested URL.
7. Run cargo build.
8. Notice that the package compiles cleanly.

Additional information

N/A