Recent changes to Babel installation altered the path in which we could

find the babel binary. The Djblets package building was updated

accordingly, and was later revised to use the future-proof symlinks for

LessCSS, UglifyJS, and Babel binaries that are placed in

node_modules/.bin/.  However, extension packaging computes paths for

these as well, and neither change updated them.
This change fixes up those paths, referencing the ones in .bin. This

should be the last time we need to modify these paths, since we're no

longer drilling into module-specific directories. With this fix,

extension packages can be built again.

Successfully built Power Pack packages.
Unit tests pass.