Check for enterprise features in the schema

[Geal]

So far, the enterprise features check only covers the presence of specific configuration option, but we're moving towards enterprise features carried by directives, so only present in the schema. We need the feature check to look at the schema and:

• detect enterprise directives, even if imported under a different name
• detect if the directives are actually used (not just present)
• refuse to load the schema if the directives are used but no valid license is found

We can have a later check in plugins to refuse to load if the feature is disabled in configuration

[Geal]

Fixed by #3713