Pass search path directly to virtualenv

Both pip and virtualenv take a path for finding packages, however the
two are separate with distinct behavior with regard to finding
packages (for perfectly valid reasons).

So, we pass the path directly to virtualenv, but pass it as a file URL
to pip.

BUG=chromium:674681
TEST=Debug on builder

Change-Id: I32f9e4e604f01387d6995230a10817f10951e9c2
Reviewed-on: https://chromium-review.googlesource.com/421183
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
diff --git a/create_venv b/create_venv
index fc148e1..3120c33 100755
--- a/create_venv
+++ b/create_venv
@@ -9,7 +9,7 @@
 set -eu
 
 basedir=$(readlink -f "$(dirname "${BASH_SOURCE[0]}" )")
-pkgdir="file://$basedir/pip_packages"
+pkgdir="$basedir/pip_packages"
 
 main() {
   if [[ $# -ne 2 ]]; then
@@ -68,7 +68,7 @@
   local venv_dir=$1
   local requirements=$2
   venv_command "$venv_dir" \
-               pip install --no-index -f "$pkgdir" -r "$requirements"
+               pip install --no-index -f "file://$pkgdir" -r "$requirements"
 }
 
 venv_command() {