[cipd] Match version to vars value supporting in gclient.py

For now 'version' of packages only support string like 'version:27.0.5'.
It is helpful to support version to match vars value, e.g.
Var('tool_version'), especially useful when version numbers could be
modified via api. This change won't affect versions with string format.

Change-Id: I40b389b528f04712798b08f1455869cab2ec54c9
Reviewed-on: https://chromium-review.googlesource.com/935666Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Shenghua Zhang <shenghuazhang@chromium.org>

gclient.py

@@ -718,6 +718,11 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
               # should suffice.
               'https://chrome-infra-packages.appspot.com')
         for package in dep_value.get('packages', []):
+          if 'version' in package:
+            # Matches version to vars value.
+            raw_version = package['version']
+            version = raw_version.format(**self.get_vars())
+            package['version'] = version
           deps_to_add.append(
               CipdDependency(
                   self, name, package, cipd_root,