rewrite profiles

This removes profile subdirs of `_build` but everything else regarding profile options is the same.

One issue I realized this has is `default` deps take precedence. We probably want the precedence of deps to be based on the order the profiles are added to the list of profiles. 

For example, `REBAR_DEFAULT_PROFILE=prod rebar3 ct` would have `[default, prod, test]` and dependencies from prod would override those from default, and those from test override prod and default.

Does that sound right?