Broccoli Plugin Needs to clean up block files on its own

After the plugin is run, there should be no Block files left in the tree. Getting all the transitive deps from our analysis should be enough to resolve all block file paths, but the identifier is wrong for some reason right now...

https://github.com/css-blocks/css-blocks/pull/90/files/0063df33f4a4b08659bad3ef96d7277cff1821d4#diff-adb5d603eb48c1343ceab22b9634c091R77

Start with the Broccoli plugin and the referenced PR #90 diff, then trace the transitive-dependency analysis that resolves Block file paths and inspect the identifier currently used there. The work is done when running the plugin leaves no Block files in the tree.

Written by the indexing model from the issue text.