Services should be exported at the main export

[bevacqua]

Today, we access services via '@elastic/eui/lib/services'. This isn't ideal, as it forces consumers to remember the path, ties the interface up to internal implementation details (directory structure), and murks the distinction between what constitutes public interface and the EUI implementation.

We should expose services such as the htmlIdGenerator function at the top level, so that it can be imported just like any component:

import {
  EuiButton,
  EuiText,
  htmlIdGenerator,
} from '@elastic/eui'

[bevacqua]

cc @cjcenizal

[chandlerprall]

Services have been exposed to the top level