ContentFiles: Embedded resources are always flattened

When a NuGet package is created with content files in a nested directory structure, the (manifest) resource names of the embedded resources that are created for these files do not contain the relative directory path (with slashes converted to dots).

### Steps to reproduce:

1. Create a **NuGet** package with the following content:

```
contentFiles
any
any
sub
myResource.txt
```

2. In the **.nuspec** for this package, add the following fragment to the metadata element:

```xml



```

3. Pack & Publish the **NuGet** package.
4. Create (for example) a **C# Console App** project named **ConsoleApp1** (with default namespace **ConsoleApp1**¨) in Visual Studio (15.8).
5. Ensure that the default package management format is **PackageReference** in the *NuGet Package Manager Settings**.
6. Add a reference to the **NuGet** package that we published in step **3**.
7. Build the project.

### Expected result:

The **myResource.txt** file is embedded in the **ConsoleApp1.exe** assembly with the name 'ConsoleApp1.sub.myResource.txt'.

### Actual result:

The **myResource.txt** file is embedded in the **ConsoleApp1.exe** assembly with the name 'ConsoleApp1.myResource.txt'.

NuGet product used: VS UI (15.8.1)
OS version: Windows 7 Build 7601

Moved from: https://github.com/NuGet/Home/issues/7255

cc @drieseng @tmeschter