Align types location for metro-source-map entry point

Summary:
Follow-up to D44509764. `metro-source-map`'s entry point is `src/source-map.js`.

Changelog: **[Types]** Fix TypeScript types entry point for metro-source-map

Reviewed By: jacdebug

Differential Revision: D44953195

fbshipit-source-id: 82aafbe443f51ae1f1f2dffe7d4ce4ea206fa00f

tsconfig.json

@@ -8,7 +8,11 @@
     "paths": {
       "metro": ["./packages/metro/types"],
       "metro/src/*": ["./packages/metro/types/*"],
-      "metro-*": ["./packages/metro-*/types"]
+      "metro-*": ["./packages/metro-*/types"],
+      // Mappings for main entry points which are not "src/index.js". Required
+      // only for this tsconfig - external consumers will read the "main" field
+      // and find the adjacent typedef file.
+      "metro-source-map": ["./packages/metro-source-map/types/source-map.d.ts"]
     }
   }
 }