Reapply gclient.py refactor for the third time.

Reapply gclient.py refactor for the third time.

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.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=53205

[M-A Ruel, 2010-07-12 20:13:09]

The interesting diff is gclient.py patchset 4 against patchset 2. The rest is
the same code as the previous tries. These changes were made to fix the issues
tested with the new smoke tests in GClientSmokeFromCheckout. I sent the smoke
tests reviews to tony@chromium.org to reduce the load on you since he didn't add
smoke tests in the first place when he implemented File() support.

As usually, I'll continue to monitor breakpad for python in case anything goes
wrong again but I've tested WebKit syncing correctly this time.

[Mandeep Singh Baines, 2010-07-13 20:26:14]

LGTM