Decide on best behavior for `Files` with relative paths

[barnson]

Task

Relative paths in Files/@Include change behavior based on whether there is an unnamed bind path. That's annoying. Pick one and deprecate the other in WiX v7.

See #9084 and others.

[barnson]

I'd really like to default to always using the first unnamed bindpath because it's a common use case and results in the simplest authoring.

However...

The "first" unnamed bindpath is not defined and might change, for example, based on how ProjectReferences are added to the .wixproj. That behavior change on adding another ProjectReference is exactly what we're trying to avoid, even if it's "less evil" than changing from relative to the source path.

I have been, reluctantly, forced to conclude that the least worst option is to always default relative paths as relative to the source file. That's always valid and the behavior doesn't change based on "external" changes.

[barnson]

Document breaking change too.