depgraph: Ignore blockers when computing virtual deps visibility

If a `virtual/` package declares a blocker to a package that doesn't
exist, then `_virt_deps_visible` incorrectly marks the virtual as
"not visible". Blockers should be ignored when performing the deps
visibility test since blockers are handled later in the pipeline.

Change-Id: I79786aaf832b13525f8162992862bb3996e3239e

lib/_emerge/depgraph.py

@@ -6022,6 +6022,8 @@ def _virt_deps_visible(self, pkg, ignore_use=False):
 
         for atoms in rdepend.values():
             for atom in atoms:
+                if atom.blocker:
+                    continue
                 if ignore_use:
                     atom = atom.without_use
                 pkg, existing = self._select_package(pkg.root, atom)