Previously, if a single source file was added to multiple file sets via
multiple `target_sources` calls, only the first call originally adding
it to the target was stored in the backtrace graph and referenced by
index in the singular "backtrace" field.
Add a "backtraces" field expanding this to cover all command
invocations which associate the given source file with that target.
Preserve the original "backtrace" field for compatibility. Update
spellings in the test suite to check against both fields.
Issue: #27035
9f4cf1cef7