Use index sitemap name as prefix in split sitemaps

[jameshalsall]

Description

When generating large sitemaps which result in a single index sitemap and several smaller split sitemap files, the split sitemap files do not use the same name prefix as the parent.

For example, if you have a sitemap called products.xml then the split sitemap files are named sitemap-1-1.xml, sitemap-1-2.xml etc. which does not make any sense.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

1. Create sitemap in admin named products.xml
2. Set the URL limit for the sitemap to a small number to cause split files (Stores > Configuration > Catalog > XML Sitemap > Sitemap File Limits > Maximum No of URLs Per File)
3. Generate the sitemap
4. Should generate a single index sitemap file named products.xml and several split sitemap files prefixed with products-, e.g. products-1-1.xml

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @larsroettig, thank you for the review.
ENGCOM-1377 has been created to process this Pull Request

[magento-engcom-team]

Hi @jameshalsall. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.5 release.