Replace a few os.path.join() calls to use self.checkout_path instead.

Fix SVN pack.

TEST=smoke tests
BUG=none

Review URL: http://codereview.chromium.org/3309005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58519 0039d316-1c4b-4281-b951-d872f2087c98