Bring some OOP and sanity to gclient.py.

- Changed the algorithm from breadth-first to depth-first.
- Added infinite recursion support.
- Fixed From() of From() dependency.
- Fixed cross solution custom deps aliasing.
- Removed support code for old .gclient_entries format.
- Removed IsGitCheckout() in favor for a direct check for .git presence.

TEST=all test pass

The main difference with r51760 is that new smoke tests have been added since which replicates how webkit checkouts with gclient.

Review URL: http://codereview.chromium.org/2917009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@53205 0039d316-1c4b-4281-b951-d872f2087c98