Use 2019.1's .asmdef GUID referencing

[bddckr]

This will prevent any errors we or our users may potentially get if we rename our Assembly Definition Files.

Editor: Added support for referencing Assembly Definition Files with GUIDs instead of assembly names. Enable with "Use GUIDs" option in Assembly Definition File inspector. Enabled by default for new Assembly Definition Files. https://unity3d.com/unity/beta/2019.1.0b3