Combine package.json and target/frontent/package.json

We should make package.json fully generated by flow and move the information in target/frontend/package.json into package.json so that the plugin handles all dependencies in one place.

This would mean that any user modification directly on the package.json file might be lost on an update by flow, but it would make the developers life easier as then the knowing which modules the project is actually depending on is easier and keeping track on versions can simply be done in version control.

Also this will simplify our code as we do not need to do hashes for dependencies to know if we need to run npm i after a mvn clean

Part of #6966

[caalador]

This would be acceptable overhead, and still a lot simpler than the current setup. To also satisfy #6907 at the same time would be to add the full object

"vaadin": {
  "dependencies": {},
  "devDependencies": {}
}

Then we could accept manual changes to devDependencies also with minimal changes in functionality.

[caalador]

Acceptance criteria:

• target/frontend/package.json is removed and all dependencies are added to package.json
• package.json gets a new JsonObject vaadin that contains dependencies and devDependencies that are the framework managed packages.
• managed packages in vaadin should accept newer versions by users to dependencies, but override too old versions
• Framework should not touch dependencies that are not found in vaadin dependencies.