IAP store package is missing reference assemblies causing DLL errors on reimporting IAP store packages

Steps To Reproduce:

1. Create a new project
2. Window > Services > Select an organisation to set up
3. IAP > Enable > IAP package should import
4. In IAP welcome page > Import store package
5. Install Unity IAP Installer > Import both iterations in Unity IAP installer
6. After importing, click on "Close, and clean up installer scripts"
7. In IAP Welcome page, Click Reimport > Install through Unity IAP installer window

Actual result: DLL errors are thrown in console

Reproducible On: 2020.1.0a19, 2019.3.0f6, 2019.2.0f2, 2019.1.14f1

Not Reproducible On: 2019.1.13f1

Note: Importing IAP store packages in 2019.3.13f1 threw a different error which was fixed in 2019.1.14f1

Environment: Win and Mac

Stack Trace:

1. Assembly 'Assets/Plugins/UnityPurchasing/Bin/Stores.dll' will not be loaded due to errors:
Unable to resolve reference 'UDP'. Is the assembly missing or incompatible with the current platform?
2. Assembly 'Assets/Plugins/UnityPurchasing/Bin/Editor.dll' will not be loaded due to errors:
Unable to resolve reference 'UDP'. Is the assembly missing or incompatible with the current platform?
Reference validation can be disabled in the Plugin Inspector.
3. Assembly 'Library/ScriptAssemblies/Assembly-CSharp-firstpass.dll' will not be loaded due to errors:
Reference has errors 'Stores'.
4. Assembly 'Library/ScriptAssemblies/Assembly-CSharp-Editor-firstpass.dll' will not be loaded due to errors:
Reference has errors 'Assembly-CSharp-firstpass'.