Add detached source lookup to results and errors

internal/engine/diagnostic.go

@@ -50,6 +50,16 @@ func (e *LoadError) Sources() []SourceInfo {
 	return cloneSourceInfos(e.sources)
 }
 
+// Source returns a detached record from the source evidence retained before
+// loading failed. IDs are local to the failed load. Empty or unknown IDs return
+// false.
+func (e *LoadError) Source(id string) (SourceInfo, bool) {
+	if e == nil {
+		return SourceInfo{}, false
+	}
+	return lookupSourceInfo(e.sources, id)
+}
+
 // WriteDiagnostics writes diagnostics in canonical order. It escapes terminal
 // control characters and propagates writer errors, including short writes.
 func WriteDiagnostics(output io.Writer, diagnostics []Diagnostic) error {