Avoid unnecessary *Ref element for Fragment definitions.

[oleg-shilo]

Discussed in #1639

^{Originally posted by Lindsay-Mathieson September 15, 2024}
Have the main installer sorted - does my desktop app and service ...

The discussion helped to identify the issue with the XML root Fragment elements (e.g. DotNetCompatibilityCheck) failing because of the unconditional *Ref element being inserted in the XML doc while it's only required for the non-root elements.

In the future even more explicit control might be required over project.AddWixFragment(...) but until the need is detected/confirmed a simple conditional auto-insert for *Ref is sufficient.