Introducing import policies

To be able to alter the import behavior, the notion of import policies introduced in 8.x-3.x and been made extendable with plugins declared with YAML files.

You can declare new import policies with YAML files and then set it on the import configurations.

Then your custom plugins/code can use theses policies to act as you want on your websites and during the imports.

See the two features already using this concept:

• #3082682: Prevent editing of shared entity on the client.
• #2975806: Allow an entity to be updated locally after being synchronized once