Issue description

Since the introduction of the serverless-appsync-plugin as a native plugin in #13217 . Our schema definitions are empty after packaging. I have tried numerous combinations of paths etc. in attempts to get it to recognise our schema files to no avail.

Our file structure is as follows (file names redacted):

- Backend
  - Core
    - serverless.yml
    - Schema
      - schema-file.graphql
      - schema-file2.graphql
  - Other service
  - serverless-compose.yml

Our serverless.yml appsync definition is as follows:

appSync:
  schema:
    - Schema/*.graphql

The output in our packaged CFN template is fine for all AppSync resources apart from the Schema, which comes out empty:

    "GraphQlSchema": {
      "Type": "AWS::AppSync::GraphQLSchema",
      "Properties": {
        "Definition": "",
        "ApiId": {
          "Fn::GetAtt": [
            "GraphQlApi",
            "ApiId"
          ]
        }
      }
    }

This has always worked with the plugin, and now seems to not work no matter whether we use the plugin or the native integration since the addition of the plugin as native.

Context

No response