refactor: Move entry points to src/

This moves our source entry points from the root folder to the src/
folder. This has the following benefits:

1. Simpler rollup config, as src/ and dist/ will have the same resulting
   structure.
2. Allows it to switch to consume the source of GitLab UI more easily.
   We could for example simply tell webpack in GitLab to replace imports
   from `@gitlab/ui/dist` with `@gitlab/ui/src`.

BREAKING CHANGE: If you consumed GitLab UI from source (e.g.
`@gitlab/ui/index.js`, you need to adjust the import to
`@gitlab/ui/src/index.js`