Hi Kris,
Thank you for providing to us a copy of your test project privately and for testing with our sample app.
I downloaded and tested your test project on my end (using Unity 2017.3.1f1 (64-bit) as well) and I was able to import the GoogleMobileAdsUnityAdsMediation.unitypackage and complete the Android Dependency Resolver successfully.
Given that you were able to import the GoogleMobileAdsUnityAdsMediation.unitypackage in our sample app on your end and we were able to import the same unity package to your test project on our end, the issue seems to be nothing specific to the Google Mobile Ads Unity plugin nor the unitypackage.
I suggest that you have this double checked by the experts over at the Unity developer forums and see if there are any similar scenarios as yours.