Support for Multi-Module projects .nuspec Dependencies

[martin1yness]

Problem:
Unable to link .NET library projects together into published NuGet package

Summary:
Common processes in Java projects build with Gradle/Maven include multi-module solutions deploying multiple JARs linked together by a POM.XML. This dependency management feature is mirrored in NuGet but is not supported by the NuGet gradle plugin.

Proposal:
Add support for dependencies in the .NuSpec: https://docs.microsoft.com/en-us/nuget/reference/nuspec#dependencies by searching all dependent project references that apply the nuget plugin. Look up artifact's nuget publishing metadata and use to define as same version dependency in the .nuspec

[timotei]

@martin1yness If it help, you can set the nuspecFile to an existing (base) spec file and it will use that info for the generated nuspec.