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

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

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.

--- a/mercurial/subrepo.py	Thu Mar 02 13:30:28 2017 +0100
+++ b/mercurial/subrepo.py	Thu Mar 02 13:30:38 2017 +0100
@@ -35,6 +35,7 @@
     pycompat,
     scmutil,
     util,
+    vfs as vfsmod,
 )
 
 hg = None
@@ -615,7 +616,7 @@
     def wvfs(self):
         """return vfs to access the working directory of this subrepository
         """
-        return scmutil.vfs(self._ctx.repo().wvfs.join(self._path))
+        return vfsmod.vfs(self._ctx.repo().wvfs.join(self._path))
 
     @propertycache
     def _relpath(self):
@@ -677,7 +678,7 @@
 
     @propertycache
     def _cachestorehashvfs(self):
-        return scmutil.vfs(self._repo.join('cache/storehash'))
+        return vfsmod.vfs(self._repo.join('cache/storehash'))
 
     def _readstorehashcache(self, remotepath):
         '''read the store hash cache for a given remote repository'''