[BUG] Errors when running `nuget pack`

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
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Description

Got the following errors when running nuget pack -Build -OutputDirectory out src/Open.Petstore/Open.Petstore.csproj (package name set to Open.Petstore)

  Model/Order.cs(37,10): error CS0246: The type or namespace name 'JsonConverterAttribute' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Order.cs(37,10): error CS0246: The type or namespace name 'JsonConverter' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Order.cs(37,31): error CS0246: The type or namespace name 'StringEnumConverter' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Pet.cs(69,10): error CS0246: The type or namespace name 'JsonConstructorAttributeAttribute' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Pet.cs(69,10): error CS0246: The type or namespace name 'JsonConstructorAttribute' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Client/OpenAPIDateConverter.cs(19,41): error CS0246: The type or namespace name 'IsoDateTimeConverter' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Client/ExceptionFactory.cs(23,67): error CS0246: The type or namespace name 'IRestResponse' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Pet.cs(37,10): error CS0246: The type or namespace name 'JsonConverterAttribute' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Pet.cs(37,10): error CS0246: The type or namespace name 'JsonConverter' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]
  Model/Pet.cs(37,31): error CS0246: The type or namespace name 'StringEnumConverter' could not be found (are you missing a using directive or an assembly reference?) [/private/tmp/Code/csharp/openapi_client/src/Open.Petstore/Open.Petstore.csproj]

    3 Warning(s)
    44 Error(s)

Time Elapsed 00:00:05.19
Error NU5013: Failed to build 'Open.Petstore.csproj'.

openapi-generator version

Latest master

OpenAPI declaration file content or url

Pestore spec (OAS 2.0)

Command line used for generation

nuget pack -Build -OutputDirectory out src/Open.Petstore/Open.Petstore.csproj

A workaround is to use nuget pack src/Open.Petstore/Open.Petstore.nuspec instead.

Reproduce the failure with nuget pack -Build -OutputDirectory out src/Open.Petstore/Open.Petstore.csproj and compare it with packing src/Open.Petstore/Open.Petstore.nuspec. Inspect the generated Open.Petstore.csproj, Model/*.cs, Client/OpenAPIDateConverter.cs, and Client/ExceptionFactory.cs; done means the project builds and packages without the reported missing-type errors.