ES Modules should respect <base> tag

[Guy Bedford]

When using the <base href> tag, the value should affect the resolution of ES modules when resolving top-level module scripts with relative imports.

For example:

```
<base href="/modules/" />
<script type="module">
import './module.js';
</script>
```

should load `/modules/module.js` and not `/module.js`.

[Guy Bedford]

Correction: the problem is specifically for dynamic imports:

```
<base href="/modules/" />
<script type="module">
import('./module.js');
</script>
```

where the top-level parent for the dynamic import should still respect the base.

[Radar WebKit Bug Importer]

<rdar://problem/55361194>

[Yusuke Suzuki]

The problem is that import is using SourceOrigin information. We should add a flag to that like, "this source is inlined script etc.".