eszip build fails with: Error: Blocked by null entry for ""https://deno.land/x/""

Hello!

When I attempt to build an eszip with the following importMap and a very simple module. I receive the error below.

**mod.ts**
```ts
import 'std/fs/mod.ts';
import 'https://deno.land/x/fresh/server.ts'
```

**importMap.json**
```json
{
"imports": {
"std/": "https://deno.land/std/",
"https://deno.land/x/": "./vendor/deno.land/x/"
}
}
```

```
error: Error: Blocked by null entry for ""https://deno.land/x/""
const ret = new Error(getStringFromWasm0(arg0, arg1));
^
at __wbg_new_651776e932b7e9c7 (file:///workspaces/eszip/lib/eszip_wasm.generated.js:313:19)
at (wasm://wasm/00a90a36:1:78732)
at (wasm://wasm/00a90a36:1:1463894)
at (wasm://wasm/00a90a36:1:1957066)
at __wbg_adapter_18 (file:///workspaces/eszip/lib/eszip_wasm.generated.js:144:6)
at real (file:///workspaces/eszip/lib/eszip_wasm.generated.js:128:14)
```

Reproduce the failure using the mod.ts and importMap.json examples, then inspect the stack entry in lib/eszip_wasm.generated.js. Verify how the import-map entry for https://deno.land/x/ is handled during the eszip build; done means the example builds without the null-entry error and resolves the imports.

Written by the indexing model from the issue text.