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: I0ac05c80a1d5fc1b8f8bd2c5ca5dabdfeeecbe52
1 file changed