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.

setup-gradle/action.yml

@@ -143,8 +143,8 @@ outputs:
 
 runs:
   using: 'node20'
-  main: '../dist/main/index.js'
-  post: '../dist/post/index.js'
+  main: '../dist/setup-gradle/main/index.js'
+  post: '../dist/setup-gradle/post/index.js'
 
 branding:
   icon: 'box'