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
The `fileSetIndex` field of target sources replies is incomplete, as a
single source file can be added to arbitrarily many file sets. Add a
`fileSetIndexes` field to report all file sets to which a source
belongs, and preserve `fileSetIndex` for compatibility as the index of
the most recent addition.
Tweak the test suite to use the plural spelling only in expected JSON
snippets and verify that the singular `fileSetIndex` is always the last
element of the new list. (Note that remaining occurrences of the
singular spelling correspond to "installers" objects, which is entirely
different.)
Fixes: #27035
Tyler Yankee
Craig Scott
Vito Gamberini
Kyle Edwards