Add type signature for TargetContents.go
This is a fairly trivial change, but it made it a bit easier for me to work out what was going on in the TargetContents test (one of the ones I have broken while working on #19439 (closed)).
This is a fairly trivial change, but it made it a bit easier for me to work out what was going on in the TargetContents test (one of the ones I have broken while working on #19439 (closed)).