@gridsome/source-graphql and wp-graphql-woocommerce incompatible

## Description
When wp-graphql-woocommerce is activated, the app has error building. The error disappears only if disable the wp-graphql-woocommerce or comment out @gridsome/source-graphql part.

BUT direct requests to the site, for example via postman works ok.

Any body can play with "https://jamstack-wp.space/graphql" - it open url.

gridsome.config.js
```
module.exports = {
siteName: 'Gridsome',
templates: {
MediaPost: '/media/:slug',
},
plugins: [
{
use: '@gridsome/source-graphql',
options: {
url: 'https://jamstack-wp.space/graphql',
fieldName: 'wpGraphQl',
typeName: 'wpGraphQl',
},
},
{
use: '@gridsome/source-filesystem',
options: {
typeName: 'MediaPost',
path: './content/media/*.json',
},
},
],
}
```

### Steps to reproduce

1. WORDPRESS
- install WooCommerce, activate, setup
- install WP GraphQL, activate, setup
- install WPGraphQL WooCommerce (WooGraphQL), activate
2. GRIDSOME app
- add @gridsome/source-graphql plagin to app, setup it
- npm run develope

### Expected result
app started in development mode

### Actual result
Error

### Environment
"@gridsome/source-filesystem": "^0.6.2",
"@gridsome/source-graphql": "^0.1.0",
"@gridsome/transformer-json": "^0.2.1",
"gridsome": "^0.7.0",

WordPress: 5.6
WooCommerce: 4.8.0
WP GraphQL: 1.0.2
WPGraphQL WooCommerce (WooGraphQL): 0.7.0

Start with the gridsome.config.js configuration and reproduce the build with @gridsome/source-graphql enabled alongside WPGraphQL WooCommerce. Compare the failing build with direct requests to the supplied GraphQL endpoint; done means the Gridsome app starts in development mode with both integrations active.

Written by the indexing model from the issue text.