Issue title: Project includes do not handle soft symlinks on linux, instead copy entire linked directory
 Issue body: If I have something like the following in my project file:

```csproj
<None Include="fixtures\**" CopyToOutputDirectory="PreserveNewest" LinkBase="fixtures\" />
```

And then inside of fixtures I have a symlinked folder I created using `ln -s [source] [link]`. But the problem is when I build the project dotnet removes the symlink and instead just copies everything over. This doesn't work as when running in debug I need the folder to be elsewhere on the system, both because of access and because it's very large.

Is there a workaround for this?

I put linux in the title, although I have not tested it on any other operating system.
 Comments: 
 Comment 0: I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one [area label](https://github.com/dotnet/runtime/blob/master/docs/area-owners.md).
{'source': 'GithubIssues', 'repo_name_with_owner': 'dotnet/runtime', 'issue_id': 904048626, 'repo_stars': 10640.0}