I was able to get this working in my Android Silver project. As the last message says, use an explicit version number, like so:
<GradleReference Include="com.android.support.constraint:constraint-layout:1.0.2" />
I also noticed that the constraint layout jars are not put in a standard search location in the sdk folder. My guess is that Fire is not scanning the actual folder it is in. On my macOS system, it is laid out like so:
What I did was copy/paste from the /extras/m2repository/com/android/support/constraint into /extras/android/m2repository/com/android/support and then everything worked for (and as a bonus, it shows up in the Gradle Reference list in the Fire UI).