Use sub-directory for 'setup-gradle' dist

To prepare for converting the 'dependency-submission' action into Typescript,
we move the 'setup-gradle' entry points and outputs into a sub-directory.

sources/tsconfig.json

@@ -59,5 +59,6 @@
     // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
     // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
   },
+  "include": ["src/**/*.ts"],
   "exclude": ["node_modules", "**/*.test.ts"]
 }