This removes the need to copy-paste code that reads a specific directory
in Program.cs.
Updated docs/samples and tests.
We need a workaround in tests to be able to use a P2P for the library. What
I did here is the same trick we do in Razor.
- Force everyone to use an MSBuild variable to locate the library
- Set that variable in a Directory.Build.props for the normal build
- Drop a special Directory.Build.props for testing
Updates a bunch of test code to use new helper methods for copying
stuff, so we can correctly do this.