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

1. ifdef-ed code from EntityFramework.Design assembly that doesn't compile with core CLR.
2. Replaced Assembly.LoadFile with Assembly.Load and removed the References option.
3. Replaced all variable names like assemblyFile with assemblyRef.
4. Using PathResolver to resolve relative paths instead of appending to GetCurrentDirectory.
5. Updated code DbContextConfigurationExtensions.GetMigrationNamespace to cover the case of empty context namespace.
6. Updated tests.
This reference should be under the k10 framework. (I'll fix in a future PR.)