[BUG] csharp-refactor generated client doesn't build in .net core but works with csharp generator

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
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• What's the actual output vs expected output?
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Description

Generated a client using csharp-refactor and it doesn't build in .NET Core. Getting following errors:

Microsoft (R) Build Engine version 15.8.166+gd4e8d81a88 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restoring packages for /Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj...
  Installing Newtonsoft.Json 12.0.1.
  Installing JsonSubTypes 1.5.1.
  Generating MSBuild file /Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/obj/Org.OpenAPITools.csproj.nuget.g.props.
  Generating MSBuild file /Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/obj/Org.OpenAPITools.csproj.nuget.g.targets.
  Restore completed in 3.26 sec for /Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj.
Api/ActivitiesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CalculationsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CategoriesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CountriesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/HealthCheckApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ImportsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ItemsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ItemSelectorsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/PriceListsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Client/ApiClient.cs(35,48): error CS0234: The type or namespace name 'Serializers' does not exist in the namespace 'RestSharp' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Client/ApiClient.cs(35,83): error CS0234: The type or namespace name 'Deserializers' does not exist in the namespace 'RestSharp' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]

Build FAILED.

Api/ActivitiesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CalculationsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CategoriesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/CountriesApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/HealthCheckApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ImportsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ItemsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/ItemSelectorsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Api/PriceListsApi.cs(17,18): error CS0234: The type or namespace name 'Mime' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Client/ApiClient.cs(35,48): error CS0234: The type or namespace name 'Serializers' does not exist in the namespace 'RestSharp' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
Client/ApiClient.cs(35,83): error CS0234: The type or namespace name 'Deserializers' does not exist in the namespace 'RestSharp' (are you missing an assembly reference?) [/Users/i52017/dev/openapi-gen/output/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
    0 Warning(s)
    11 Error(s)

Time Elapsed 00:00:04.35

openapi-generator version

4.0.0-beta

OpenAPI declaration file content or url

https://api.xactprm.com/pricing/swagger/current/swagger.json

Command line used for generation

openapi-generator generate -i swagger.json -g csharp-refactor -c config.json -o output

Config file:

{
    "targetFramework": "v5.0",
    "netCoreProjectFile": true
}

Steps to reproduce

1. Generate code
2. cd output
3. dotnet build

Reproduce the issue with OpenAPI Generator 4.0.0-beta using the supplied Swagger URL, csharp-refactor generator, config.json, and dotnet build. Inspect the generator output referenced in Api/ActivitiesApi.cs and Client/ApiClient.cs, then trace the csharp-refactor templates and dependency configuration; done means the generated .NET Core project builds without the System.Net.Mime and RestSharp namespace errors.