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>