Add support for placeholder dependency indicating empty dependency group.
Add support for placeholder dependency indicating empty dependency group.

Sample usage:
```
<Dependency Include="_._">
  <TargetFramework>net46</TargetFramework>
</Dependency>
```

An empty dependency group is needed when you support a platform but don't need any dependencies but another group (for example an earlier framework) required dependencies.

zbrad/nuproj,nuproj/nuproj,kovalikp/nuproj,NN---/nuproj