Codegen does not work with fragments from external package

The issue is apollo codegen throw `UnkonwFragment` error if fragment is imported from other package.

**Intended outcome:**

One of my monorepo packages exports fragment field on exported react component like here:

```jsx
// monorepo/packages/Email/src/index.tsx
import gql from 'graphql-tag';

interface Props {
data: Data // generated as expected by codegen
}

const Email = ({data}: Props) => email is {data.email}

Email.fragment = gql`
fragment Email on Query {
__typename
email
}
`

export default Email
```

so after building (just ts compiler) I want to use this package:

```jsx
// monorepo/demo/src/index.tsx
import gql from 'graphql-tag';
import Email from 'email'

const App = ({data}) =>

App.fragment = gql`
fragment App on Query {
...Email
}
${Email.fragment}
`
```

I expected that runing apollo codegen in application will generate proper type for `data` prop in `App`. Regardless this code works fine and data is fetched exactly as expected.

**Actual outcome:**
Instead of proper code generation i got `unkown fragment error:

```
.../monorepo/demo/src/index.tsx: Unknown fragment "EmailPreview_data".
{ ToolError: Validation of GraphQL query document failed

```

**Versions**

```
"apollo": "^2.12.3",
"graphql": "~14.2.1",
"graphql-tag": "^2.10.1",
"react-apollo": "^2.5.6"
```

Reproduce the failure from monorepo/demo/src/index.tsx using the exported fragment in monorepo/packages/Email/src/index.tsx and the listed Apollo, GraphQL, and graphql-tag versions. Start by tracing Apollo codegen's handling of imported fragment documents; done means codegen accepts the external Email fragment and generates the proper App data prop type without an Unknown fragment error.

Written by the indexing model from the issue text.