Scope of external node classifier data

Variables provided by an ENC are set at the top scope. However, all of the classes assigned by an ENC are declared at the node scope.

This results the most expected behavior: variables from an ENC are available everywhere, and classes can use node-specific variables.

This means compilation fails if a site manifest tries to set a variable that was already set at top scope by an ENC.

Node scope only exists if there is at least one node definition in the main manifest. If no node definitions exist, then ENC classes get declared at top scope.