chore: rename aws-synthetics → aws-synthetics-alpha (#25055)

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.

----

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

@@ -16,7 +16,7 @@ coverage
 nyc.config.js
 !.eslintrc.js
 !jest.config.js
-!test/canaries/nodejs/*
+!test/canaries/**/*
 
 junit.xml
 

packages/@aws-cdk/aws-synthetics/.gitignore renamed to packages/@aws-cdk/aws-synthetics-alpha/.gitignore

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