Move private modules into the -private directory

[bmac]

to discourage importing from paths that may change in future versions

cc @fivetanley @stefanpenner @rwjblue

This pr moves everything but index.js into the -private directory. We can make individual prs to move stable modules out of private directory when we decide they are stable.

[rwjblue]

👍

[fivetanley]

RESTAdapter is public API though right?

[bmac]

Right now in Ember Data 2.2.0 the public api to access any Ember Data module is:

import DS from 'ember-data';

// then do something with `DS.RESTAdapter`

Instead of trying to make a judgement call about what we should allow users to import and what we should keep hidden I decided to move everything into the -private module path. After this is merged we can open new prs to move things like the RESTAdapter to a public path. That way we can be more deliberate and discerning about what gets exposed publicly.

[rwjblue]

Another way of putting this: to date Ember Data's public API has been strictly the globals output (which seems unchanged by this PR). This PR helps ensure that we are not inadvertently making many more of our internals "public" because the modules are importable and not marked as private.