It appears that another extension, SequenceRegistration, is also having the same issue. That extension depends on Sequences and SlicerElastix. Here’s the nightly Windows build for SequenceRegistration that has the same CMake warning: http://slicer.cdash.org/viewConfigure.php?buildid=1770198.
A previous SlicerElastix error due to being a dependency was brought up in Failing Tests - ImportError: No module named Elastix, but that solution by @jcfr is evidently not working anymore.