Fix: several sitemap related issues

[moonmeister]

1. I have fixed a couple small bugs. As noted in Support for v4 of gatsby-plugin-sitemap #438 the default sitemap name has changed to (sitemap-index.xml) though I am trying to revert the default path change from /sitemap to / in fix(gatsby-plugin-sitemap): Sitemap path bug gatsbyjs/gatsby#31184. If this change probably requires a major version bump as it would be breaking.

2. Because of the above change in gatsby-plugin-sitemap I tried to set the sitemap option on my site and it would not change from the default. Reading the code I realized there was a bug caused by the if statement I've fixed this so the custom sitemap path can be set without declaring the host as well.

3. I removed the dependency on nodes old url package to make this more compatible with modern node as well.

4. I added logic to allow the custom sitemap to be a relative path and prefix the host.

5. I've put in logic to correctly handle Gatsby's pathPrefix feature.

If you'd like me to split any of this up into smaller PRS or remove anything let me know. Thanks.

Fixes #438

[mdreizin]

Hi folks! Thank you very much for contribution. It looks good to me. I am going to merge it. I am really sorry for the delayed review and response.

[mdreizin]

LGTM.

[mdreizin]

Please wait for a release in a couple of minutes, github-actions[bot] will post a comment here.

[github-actions]

🎉 This PR is included in version 1.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[josephmarkus]

This change breaks when used with gatsby-plugin-sitemap 3.3.0 with the following:

> 80 |       mergedOptions.sitemap = new URL(mergedOptions.sitemap.startsWith(pathPrefix) ? mergedOptions.sitemap : path.posix.join(pathPrefix, mergedOptions.sitemap), mergedOptions.host).toString()

12:48:39 PM:
  79 |     } catch {

12:48:39 PM:
ERROR "gatsby-plugin-robots-txt" threw an error while running the onPostBuild lifecycle:

12:48:39 PM:
  78 |       new URL(mergedOptions.sitemap)

12:48:39 PM:
Cannot read property 'startsWith' of null

12:48:39 PM:
  83 |

12:48:39 PM:
     |                                                             ^

12:48:39 PM:
  82 |   }

12:48:39 PM:
  81 |     }

12:48:39 PM:
  TypeError: Cannot read property 'startsWith' of null

12:48:39 PM:
    [www]/[gatsby]/src/utils/api-runner-node.js:434:22

12:48:39 PM:
  - gatsby-node.js:80 Object.onPostBuild

Why is this breaking change not reflected in the version of this package update?

[moonmeister]

Please see: #451 or open a new issue if appropriate

[fmaiabatista]

Hi there, I have just started using the plugin and was confused by it's output differing from the docs I found in Gatsby's website. I expected them to live in example.com/sitemap.xml but they are instead located in example.com/sitemap/sitemap-index.xml.

Can you please update the docs/README according to the latest changes? I know this is recent but it helps whoever finds themselves in the same situation as me!

Thanks,