Bug - scripts added to the serverless.yml are running twice

[sagar-cws]

This is a Bug Report

Description

There are 2 issues after plugin version 1.4.0

1. use of ${self:} throws error
   ServerlessError: Trying to populate non string value into a string for variable ${self:}. Please make sure the value of the property is a string
   using ${self} works

2. serverless plugin (version 2.29.0) does the variables.populateService on start which runs the scripts from serverless.yml
   link: https://github.com/serverless/serverless/blob/f5b2b9be395c9c2d3de4c4f91f991276bc22dc33/lib/Serverless.js#L296

also the serverless-step-functions plugin (version 2.30.0) tries to populate the variables before setting up step functions
link:

serverless-step-functions/lib/yamlParser.js

Line 26 in e0a391d

.then(serverlessFileParam => this.serverless.variables.populateObject(serverlessFileParam)

because of this scripts are running twice, which is not expected.

this is a link to a PR after which these issues are coming
#116

sharing a sample example to reproduce the issue

service: test-step-function
provider:
  name: aws
  onStart: ${file(./on_start.js):onStart}

frameworkVersion: '2'

stepFunctions:
  stateMachines:
    hellostepfunc1:
      definition:
        StartAt: HelloWorld1
        States:
          HelloWorld1:
            Type: Task
            Next: ToSqsQueue
            Resource: "arn:aws:lambda:#{AWS::Region}:#{AWS::AccountId}:function:hello-world"
            End: true
          ToSqsQueue:
            Type: Task
            Resource: arn:aws:states:::sqs:sendMessage
            Parameters:
              QueueUrl: https://sqs.#{AWS::Region}.amazonaws.com/#{AWS::AccountId}/hello-world-queue
              MessageBody.$: "$"
              MessageGroupId: "1"
            End: true

plugins:
  - serverless-step-functions

custom:
  servicename: ${self:} # first issue, ${self:} doesn't work after version 1.4.0, works for ${self}, error is ServerlessError: Trying to populate non string value into a string for variable ${self:}. Please make sure the value of the property is a string
  add_resources: ${file(./resources.js)}
  export:
    statics:
      - service: ${self:custom.servicename}

Additional Data

sample project to reproduce the issues:
step_functions_example.zip