Refactor away from explicit file paths for bourbon/neat

[SYU15]

I recently ran into an issue pulling this repo in as a git submodule in for ember-debug. The explicit filepaths for bourbon (ex. node_modules/bourbon/app/assets/stylesheets/_bourbon) do not play nicely with our yarn workspace setup since the bourbon package is added in the root directory of the yarn workspace and not at the ember-observer level.

Not sure if a solution is to add/possibly update ember-cli-bourbon to work so we can just use @import bourbon or if there was another path forward that the repo owners had in mind. Happy to help out as needed since this is probably only impacting ember-debug work.

Related PR commit here: emberjs/ember-debug@472cfbe