Moves content of core.api in core, and adds Ixxx prefix

Associated plugin branches:
https://github.com/skyluc/scala-worksheet/tree/issue/API-ScalaProject-location
https://github.com/skyluc/scala-ide-play2/tree/issue/API-ScalaProject-location
https://github.com/skyluc/scala-search/tree/issue/API-ScalaProject-location