chore: rename aws-neptune → aws-neptune-alpha (#25077)

This PR is part of a refactor to make all directory names equal to the package name of the package inside it.

It is a re-roll of #24930, but split into parts (one PR per package), so that we will cut down on merge conflicts.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/@aws-cdk/aws-neptune/.eslintrc.js renamed to packages/@aws-cdk/aws-neptune-alpha/.eslintrc.js

@@ -45,7 +45,7 @@
   "repository": {
     "type": "git",
     "url": "https://github.com/aws/aws-cdk.git",
-    "directory": "packages/@aws-cdk/aws-neptune"
+    "directory": "packages/@aws-cdk/aws-neptune-alpha"
   },
   "homepage": "https://github.com/aws/aws-cdk",
   "scripts": {