Proj0500: Only include packages with an explicitly defined license
Using a NuGet (third-party) package implies that you and/or your company explicitly agree with the legally binding conditions of the license and the copyright of the owner of the package.
As Microsoft states it itself:
If a package does not specify the licensing terms, contact the package owner directly using the Contact owners link on the NuGet.org package page. Microsoft does not license any intellectual property to you from third party package providers and is not responsible for information provided by third parties.
When you use packages that are only (privately) shared within your company those packages are advised to explicitly define a license. You might need to contact your legal department to define a proper license.
This rule can detect missing licenses in NuGet spec files, but can not be considered legal advice, nor is this documentation.