vfs: use 'vfs' module directly in 'mercurial.unionrepo'

Now that the 'vfs' classes moved in their own module, lets use the new module
directly. We update code iteratively to help with possible bisect needs in the
future.

mercurial/unionrepo.py

@@ -27,8 +27,8 b' from . import ('
     pathutil,
     pycompat,
     revlog,
-    scmutil,
     util,
+    vfs as vfsmod,
 )
 
 class unionrevlog(revlog.revlog):
@@ -39,7 +39,7 b' class unionrevlog(revlog.revlog):'
         #
         # To differentiate a rev in the second revlog from a rev in the revlog,
         # we check revision against repotiprev.
-        opener = scmutil.readonlyvfs(opener)
+        opener = vfsmod.readonlyvfs(opener)
         revlog.revlog.__init__(self, opener, indexfile)
         self.revlog2 = revlog2