PP file output is not generated during the build in classic c# project

PP file output is not being generated during the build

**Steps:**
1. create c# class library
2. Install package contentfilesexample -v 1.0.2 as PackageReference

**Expected**
ExampleReader will be in the current namespace from the pp file.

**Actual**
ExampleReader does not exist

Copied from https://github.com/dotnet/sdk/issues/1100 for classic c# project system as well