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.

No file or test is named. Reproduce the import-map example and compare the stored module keys with the WICG Import Maps specification; done means determining whether the file:///~/ prefix is required and correcting the behavior or documenting the specification basis.

Written by the indexing model from the issue text.