Use proper location ref for custom source location

packages/catalog-model/src/location/annotation.ts

@@ -17,3 +17,5 @@
 export const LOCATION_ANNOTATION = 'backstage.io/managed-by-location';
 export const ORIGIN_LOCATION_ANNOTATION =
   'backstage.io/managed-by-origin-location';
+
+export const SOURCE_LOCATION_ANNOTATION = 'backstage.io/source-location';

packages/catalog-model/src/location/index.ts

@@ -20,4 +20,8 @@ export {
   locationSpecSchema,
   analyzeLocationSchema,
 } from './validation';
-export { LOCATION_ANNOTATION, ORIGIN_LOCATION_ANNOTATION } from './annotation';
+export {
+  LOCATION_ANNOTATION,
+  ORIGIN_LOCATION_ANNOTATION,
+  SOURCE_LOCATION_ANNOTATION,
+} from './annotation';