Clarify/names

/cc @ianstormtaylor @MatthewMueller this one clarifies some of the variable names. Eventually we can probably name them shorter things, but having everything be called `path` and `file` made it very difficult to reason about which thing was actually being talked about (the entry file, the dependency defined in a file, the file in which a dependency was defined (the parent), etc.). So named them explicitly, and perhaps later we can come back and clean it up again. But in the meantime this makes it much easier to reason about :)

Depends on this one being merged first - https://github.com/duojs/duo/pull/302