Strange key storage...

Given the import map of:

```json
{
  "imports": {
    "/~/": "./lib/"
  }
}
```

One would expect the keys of import maps modules to be:

```rust
vec!["/~/"]
```

But instead, you will get:

```rust
vec!["file:///~/"]
```

I am not aware of anything in the spec that requires it to be that way, but I might be wrong.