Recursively load mojo packages

Recursively load mojo packages

Adds simple manifest file parsing to the package downloader, and recursively schedule loading dependencies of a package.

R=ben@chromium.org

Committed: https://crrev.com/852bc07ce1354a2fee8ff88b96b658ac930a4b69
Cr-Commit-Position: refs/heads/master@{#296471}

[brettw, 2014-09-24 16:56:53]

brettw@chromium.org changed reviewers:
+ ben@chromium.org

[Ben Goodger (Google), 2014-09-24 17:12:35]

lgtm

[brettw, 2014-09-24 18:27:13]

Committed patchset #2 (id:20001) to pending queue manually as 7cee7d1 (presubmit
successful).

[commit-bot: I haz the power, 2014-09-24 18:27:35]

Patchset 2 (id:??) landed as
https://crrev.com/852bc07ce1354a2fee8ff88b96b658ac930a4b69
Cr-Commit-Position: refs/heads/master@{#296471}