[BUG] variables, functions, etc are not available if splitted into files

[AhmedNourJamalElDin]

Hi,
When I split the code into multiple files, this plugin cant recognize them.

How to re-produce:
just take the stepFunctionsOffline to its own file, and reference them in serverless.yml like this:

stepFunctionsOffline: ${file(step-functions-offline.yml)}

How to fix:
don't use the contructor to get data as you do in this code:

this.functions = this.serverless.service.functions;
this.variables = this.serverless.service.custom.stepFunctionsOffline;

in this line.