"/*" as key leads to GraphQL warning

Description

If one adds in gatsby-config an option with key /* (like: '/*': "SomeStuff") this leads to the following GraphQL warning:

Name "__" must not begin with "__", which is reserved by GraphQL introspection. In a future release of graphql this will become a hard error.

Notice that it does not matter to which plugin I pass this object.

Environment

Gatsby version: 1.9.153
Node.js version: 9.4.0
Operating System: Linux, Windows

File contents:

gatsby-config.js:

  plugins: [
    {
      resolve: `gatsby-source-filesystem`,    // Just a demo, works for all plugins
      options: {
        path: `${__dirname}/src/pages`,
        name: 'pages',
        '/*': "SomeStuff"         // This leads to warning
      },
    },
  ]

Actual result

The full warning text is in #2925

Expected behavior

No warning

Steps to reproduce

1. add the key '/*' to a plugin-option

[DSchau]

This was kind of a difficult one (for me!) to debug, but I think I've traced the issue.

The error is that this line is creating the structure ___SOMETHING (or __, if the expression matches each character). For example, consider the following table:

Input	Output
/*	__
/*.js	___js
/*.html	___html

As __ is reserved for internal GraphQL stuff, herein lies the issue. I'm not really sure what the resulting node name should be but I think I've identified the issue at least. Should be easy to fix once I'm aware of how to proceed! I'll open a PR with a first shot at it, and then we can iterate from there further.