kdbush causes webpack warning

[chris-cooper]

As previously reported with earcut, kdbush is producing the following warning with webpack.

./~/cesium/Source/ThirdParty/kdbush.js
Critical dependencies:
1:479-486 This seems to be a pre-built javascript file. Though this is possible, it's not recommended. Try to require the original source to get better results.
 @ ./~/cesium/Source/ThirdParty/kdbush.js 1:479-486

I haven't noticed any runtime issues yet, but it would be good to have a better strategy for these third party dependencies such as pulling in the un-built source via an npm dependency.