fix(python): change Python namespace to aws_cdk (#13489)

The current Python namespace in v2 for the monopackage is `aws_cdk_lib`, which
means people need to update a lot of imports their source code.

`aws_cdk` is the namespace used by the v1 packages, which means people who have
already written code against v1 have less updating to do.

Unfortunately there will still be SOME updating, as some classes *used* to be in
`aws_cdk.core` but are now in `aws_cdk` directly.

Tested that the `aws_cdk_lib` package that vends `aws_cdk/__init__.py` can
still be installed without problems alongside `aws_cdk.cx_api` which vends
`aws_cdk/cx_api/__init__.py`, and others.


----

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

packages/aws-cdk-lib/package.json

@@ -63,7 +63,7 @@
       },
       "python": {
         "distName": "aws-cdk-lib",
-        "module": "aws_cdk_lib"
+        "module": "aws_cdk"
       }
     },
     "projectReferences": false