This standardizes using "backbone" instead of "Backbone" for imports,

ensuring we always use the lowercase version and matching the package

name.
The global mapping for "Djblets" has also been changed to "djblets" to

stay consistent. This isn't currently used in any imports in the

codebase, but Review Board references it as lowercase.

Successfully built the Djblets package and Review Board packages

without problems. Verified that there weren't any issues with

Backbone imports and nothing was bundled unnecessarily.