Updated the migration tool to work with K10, including core CLR:

1. ifdef-ed code from EntityFramework.Design assembly that don't compile with core CLR.
2. Replace Assembly.LoadFile with Assembly.Load and removed the References option.
3. Replace all variable names like assemblyFile with assemblyRef.
4. Using PathResolver to resolve relative paths instead of GetCurrentDirectory.
5. Updated code DbContextConfigurationExtensions.GetMigrationNamespace to cover the case of empty context namespace.
6. Updated tests.