Don't slice if parentBase has just one element.

When doing `require('ember-qunit')` this was failing because we
couldn't  find module 'isolated-container'. The issue was the
following:

 `ember-qunit` exposes `ember-qunit` which `imports isolatedContainer from './isolated-container'`

When requiring, the loader was calling `resolve` with `child` =
`./isolatedContainer` and `name` = `ember-qunit`, which would resolve
just to 'isolated-container', the reason? `"ember-qunit".split('/').slice("/")` === `[]`.