Use lowercase "backbone" instead of "Backbone" for imports.
Review Request #13219 — Created Aug. 15, 2023 and submitted
This standardizes using "backbone" instead of "Backbone" for imports,
ensuring we always use the lowercase version and matching the package
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.