[cmx] reduce validate_cmx deps to just cmx

This gets rid of the issue where a target A that has cmx files might
depend on target B, where target B has a restricted visibility list only
to A. Thus A isn't visible to validate_cmx.

Follow up to https://fuchsia-review.git.corp.google.com/c/build/+/196740

TESTED=built all layers

CF-129 #comment

Change-Id: Ic0263ce4e893d8e88d2f0573af2f31d93df545fd
1 file changed