Support multiple files in `dotnet run`

[jaredpar]

Consider the following directory layout:

- app\
  - file.cs
  - util.cs

When executing dotnet run file.cs the code inside file.cs should be able to use types / members in util.cs. Specifically:

// file.cs
Console.WriteLine(Util.GetMessage());

// util.cs
static class Util
{
  public static string GetMessage() => "Hello World";
}

When executing dotnet run file.cs this should execute and produce the output "Hello World".