TL;DR: Open Android SDK Manager and install “Extras/Android Support Repository”.

After importing the unitypackage from play-games-plugin-for-unity, the project throws an error and stopped working. The error was not helpful at all, but the warning “Unable to find dependency com.android.support:support” indicates that the “Android Support Repository” is missing. Even if the support repository is not installed, the folder “$SDK/extras/android/m2repository” is present.
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