Ensure that each instance of spotless-changelog within a multi-project build has a different tag prefix

You can apply spotless-changelog multiple times in a multi-project build ([example](https://github.com/diffplug/spotless/blob/58e53b64ee312a8f013afbdbc2bbbd1c23eae77e/gradle/changelog.gradle#L1-L22)), but each instance needs to have a distinct changelog file, as well as a distinct `tagPrefix`, otherwise you will get errors related to duplicate tags, such as #24.

It would be nice to proactively check for this condition within the plugin and warn the user. Task configuration avoidance and configure-on-demand mean that it won't always be perfect, but there are certainly some cases where we could help the user understand that it has been configured incorrectly.