chore(toolkit-lib): include @aws-cdk/cli-plugin-contract in local packaging (#457)

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

Author: mrgrain

packages/@aws-cdk/toolkit-lib/build-tools/package.sh

@@ -12,7 +12,9 @@ mkdir -p dist/js
 npm pkg set version=0.0.0-alpha.$commit
 npm pkg set dependencies.@aws-cdk/cloud-assembly-schema=$version
 npm pkg set dependencies.@aws-cdk/cloudformation-diff=$version
+npm pkg set dependencies.@aws-cdk/cli-plugin-contract=$version
 npm pack --pack-destination dist/js
 npm pkg set version=$reset
+npm pkg set dependencies.@aws-cdk/cli-plugin-contract=^$reset
 npm pkg set dependencies.@aws-cdk/cloudformation-diff=^$reset
 npm pkg set dependencies.@aws-cdk/cloud-assembly-schema=^$reset