Auto merge of #9188 - weihanglo:issue-9041, r=ehuss

Auto merge of #9188 - weihanglo:issue-9041, r=ehuss

Package ID specification urls must contain a host

Resolves https://github.com/rust-lang/cargo/issues/9041

Not sure which commit breaks this. Cargo shipped with rust 1.46 didn't unwrap on a `None` but 1.47 did. Even checkouted to 149022b1d8f382e69c1616f6a46b69ebf59e2dea (cargo of rust 1.46), it still unwrap unexpectedly.

So I ended up following the [Specification grammer](https://doc.rust-lang.org/cargo/reference/pkgid-spec.html#specification-grammar) to make sure there is a `host` in the pkgid urls.

<details>
<summary>See console output</summary>

cargo of rust 1.46
```console
$ cargo +1.46 -vV
cargo 1.46.0 (149022b1d 2020-07-17)
release: 1.46.0
commit-hash: 149022b1d8f382e69c1616f6a46b69ebf59e2dea
commit-date: 2020-07-17

$ cargo +1.46 pkgid /path
error: package ID specification `/path` matched no packages
```

cargo of rust 1.47

```console
$ cargo +1.47 -vV
cargo 1.47.0 (f3c7e066a 2020-08-28)
release: 1.47.0
commit-hash: f3c7e066ad66e05439cf8eab165a2de580b41aaf
commit-date: 2020-08-28

$ cargo +1.47 pkgid /path
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/tools/cargo/src/cargo/core/package_id_spec.rs:234:50
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

cargo on commit 149022b1d8f382e69c1616f6a46b69ebf59e2dea

```console
$ git checkout 149022b1d8f382e69c1616f6a46b69ebf59e2dea
$ cargo run -- pkgid /path
   Compiling cargo-platform v0.1.1 ([..]/cargo/crates/cargo-platform)
   Compiling crates-io v0.31.1 ([..]/cargo/crates/crates-io)
   Compiling cargo v0.47.0 ([..]/cargo)
    Finished dev [unoptimized + debuginfo] target(s) in 22.90s
     Running `target/debug/cargo pkgid /path`
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/cargo/core/package_id_spec.rs:234:50
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
</details>