sourcebundle: Avoid infinite recursion with diagnosticInSourcePackage

This wrapper type acts as an adapter to rewrite relative source paths in
diagnostics to be absolute source addresses instead, but it was
incorrectly calling its own Source method to get the starting value,
rather than calling that of its wrapped diagnostic, thereby causing
infinite recursion until stack overflow.

sourcebundle/diagnostics.go

@@ -128,7 +128,7 @@ func (diag diagnosticInSourcePackage) Severity() DiagSeverity {
 }
 
 func (diag diagnosticInSourcePackage) Source() DiagSource {
-	ret := diag.Source()
+	ret := diag.wrapped.Source()
 	if ret.Subject != nil && sourceaddrs.ValidSubPath(ret.Subject.Filename) {
 		newRng := *ret.Subject // shallow copy
 		newRng.Filename = diag.pkg.SourceAddr(newRng.Filename).String()