[scala-sttp] Fix enum is not imported correctly

[bgong-mdsol]

This PR fixes the issue #8827

Here is the openapi spec that can be used to reproduce the issue and verify the fix. This spec includes the both inline and stand-alone Enum schemas.

openapi: 3.0.3
info:
 title: standalone or inline enums
 version: '1.0'
paths:
 /state:
   get:
     operationId: getUsers
     responses:
       '200':
         description: get Users
         content:
           application/json:
             schema:
               $ref: '#/components/schemas/Users'
components:
 schemas:
   Users:
     properties:
       emails:
         items:
           type: string
         type: array
       search:
         enum:
         - email
         - full_name
         type: string
       standAloneEnum:
         $ref: '#/components/schemas/StandAloneEnum'
   StandAloneEnum:
     enum:
       - first_name
       - last_name
       - email
     type: string

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@chameleon82 @clasnake @jimschubert @shijinkui @ramzimaalej @Bouillie @wing328 @jfeltesse-mdsol @jcarres-mdsol

[wing328]

Thanks for the PR. Tested locally and the result is good with this fix:

williamcheng /tmp/scala (master) $ sbt compile
[info] welcome to sbt 1.5.0 (N/A Java 15.0.2)
[info] loading project definition from /private/tmp/scala/project
[info] loading settings for project scala from build.sbt ...
[info] set current project to openapi-client (in build file:/private/tmp/scala/)
[info] Executing in batch mode. For better performance use sbt's shell
[info] compiling 5 Scala sources to /private/tmp/scala/target/scala-2.13/classes ...
[success] Total time: 9 s, completed 17 Sep 2021, 9:38:11 am