portage_util: add VDB_PATH constant for /var/db/pkg

This is somewhat cosmetic, but in the case of cros_list_modified_packages,
avoids an expensive portage import just for the sake of this one constant.

BUG=None
TEST=CQ passes

Change-Id: I133696f449fdeeb29837ed6ff4e86419dd892352
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2111898
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/cli/deploy.py b/cli/deploy.py
index bf42e15..a389bd2 100644
--- a/cli/deploy.py
+++ b/cli/deploy.py
@@ -745,7 +745,7 @@
 
     install_attrs = {}
     for pkg in sorted_installs:
-      pkg_path = os.path.join(root, portage.VDB_PATH, pkg)
+      pkg_path = os.path.join(root, portage_util.VDB_PATH, pkg)
       dlc_id, dlc_package = _GetDLCInfo(device, pkg_path, from_dut=True)
       install_attrs[pkg] = {}
       if dlc_id and dlc_package: