Index page not included in sitemap

[YoungiiJC]

Issue Summary

Plugin does not generate src/pages/index. i.e. sitemap is missing the root url.

To Reproduce

Here's my config + gatsby-plugin-sitemap (for comparison):

{
      resolve: `gatsby-plugin-advanced-sitemap`,
      options: {
        exclude: [`/team`, `/team/`, `/dev-404-page/`, `/404/`, `/404.html`],
        query: `{
          allPublishedTeams {
            edges {
              node {
                id
                slug
                image
              }
            }
          }
        }`,
        mapping: {
          allPublishedTeams: {
            sitemap: `teams`,
          },
        },
      },
    },
    {
      resolve: `gatsby-plugin-sitemap`,
      options: {
        output: `/some-other-sitemap.xml`,
      },
    },

gatsby-plugin-advanced-sitemap generates:

gatsby-plugin-sitemap spits out:

Technical details:

• Gatsby Version: ^2.12.1
• Node Version: 14.4.0
• React / Typescript