Allow custom module paths.

[rwjblue]

This allows for custom mapping of filenames to ES6 modules.

Specifically, in Ember we exclude lib from the generated module names, this provides a simple extension point to perform that kind of customization.

[joliss]

For removing lib, if things need to be moved into place, I'd normally use broccoli-static-compiler on the tree to move things around, so that the file names come out correct.

In general, I try to keep my trees so that file paths correspond to module names, i.e. there is no top-level lib or similar.

So I'm not super-convinced by the use case, but I'd like to hear more.

[rwjblue]

Interesting, I hadn't thought of doing it that way. I'll play around with transforming as a separate step and report back.