Rename __standalone_mode__ to STANDALONE_MODE

The fix for [1] prevents from importing python variables __foo__,
as explained in [2]. Rename standalone mode flag correspondingly.

[1] https://github.com/facebook/buck/issues/182
[2] https://github.com/facebook/buck/issues/221

Change-Id: If8682278e7c91df3fc18b3d623eb43950e839448
1 file changed