Transitive imports are not identified as file attachments

[mbostock]

For example, if I have a JavaScript cell:

```js
import {foo, bar}  from "./foo.js";
```

And then foo.js imports bar.js:

export const foo = "foo";
export {bar} from "./bar.js";

And bar.js:

export const bar = "bar";

Then foo.js is correctly identified as a file attachment, but bar.js is not.

Fixed in #40.

[cinxmo]

Transitive imports are promoted as file attachments in #40. But transitive imports are not rewritten properly. for example:
in a .md file:

```js
import {bar} from "./bar.js"
```

will get rewritten as

const {bar} = await import("/_file/bar.js");

but in bar.js this will not be rewritten

import {foo} from "./foo.js"