Create flow-components artifact in platform

[mshabarov]

Rename vaadin-core-components to flow-components.

This would be a better name synced with the react-components and react-components-core because it's gonna be a publicly used artifact for setting up Vaadin project for e.g. plain servlets app. (versus existing vaadin-core which is shorter, but gonna be deprecated in favor of "starter" dependencies).

React core components are not JAR dependencies and added by Flow separately from maven structure.