…mpilation unit

Currently, the `ng_module` rule incorrectly uses manifest paths for
generated imports from the Angular compiler.

This breaks packaging as prodmode output (i.e. `esnext`) is copied in
various targets (`es5` and `es2015`) to the npm package output.

e.g. imports are generated like:

_node_modules/my-pkg/es2015/imports/public-api.js_
```ts
import * as i1 from "angular/packages/bazel/test/ng_package/example/imports/second";
```

while it should be actually:

```ts
import * as i1 from "./second";
```

The imports can, and should be relative so that the files are
self-contained and do not rely on custom module resolution.