Take a look at Crossbill LicenseNoticeAggregator on Github.

The tool iterate through NuGet packages and get the licensing notices in a single file for a product release. It respects the different way the licensing information saved in NuGet package including HTML to plain text conversion.

You can call if from a command line interface, from a PowerShell script or form a VisualStudio project. Mostly the task can be automated, but a small amount of packages have to be processed manually and put as a text file in a tool's directory for processing.

Disclaimer: I am the author of the tool.