chore(aws-cdk-lib): change namespaces/package names in line with RFC 6 (#13494)

Changes:

* .NET: Namespace changed from `Amazon.CDK.Lib` -> `Amazon.CDK` (so `Stack` has the same FQN, same namespace as in Monocdk)
* Java: Package name changed from `software.amazon.awscdk.lib` -> `software.amazon.awscdk.core` (so `Stack` has the same FQN, same namespace as in Monocdk)
* Java: Changed artifact ID to match what's written in [RFC 6]
* Python: Changed dist name to match what's written in [RFC 6]
* Python: Change namespace to `aws_cdk` instead of `aws_cdk_lib` for minimal interference. Still need to test whether it's okay to change this to `aws_cdk.core` (like for Java) so `Stack` will keep the same FQN. Monocdk does something different for Python.

[RFC 6]: https://github.com/aws/aws-cdk-rfcs/blob/master/text/0006-monolothic-packaging.md

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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-lib/package.json

@@ -50,22 +50,22 @@
     "outdir": "dist",
     "targets": {
       "dotnet": {
-        "namespace": "Amazon.CDK.Lib",
+        "namespace": "Amazon.CDK",
         "packageId": "Amazon.CDK.Lib",
         "iconUrl": "https://raw.githubusercontent.com/aws/aws-cdk/master/logo/default-256-dark.png",
         "versionSuffix": "-devpreview"
       },
       "java": {
-        "package": "software.amazon.awscdk.lib",
+        "package": "software.amazon.awscdk.core",
         "maven": {
           "groupId": "software.amazon.awscdk",
-          "artifactId": "lib",
+          "artifactId": "aws-cdk-lib",
           "versionSuffix": ".DEVPREVIEW"
         }
       },
       "python": {
-        "distName": "aws-cdk.lib",
-        "module": "aws_cdk.lib"
+        "distName": "aws-cdk-lib",
+        "module": "aws_cdk"
       }
     },
     "projectReferences": false