[Icon Builder] Move to packages directory

[mbrookes]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

Following the recent discussion, this PR moves icon-builder to the proposed packages directory.

[newoga]

Maybe we should rename the folder to material-ui-icons or rename the package.json package name to material-ui-icon-builder. And maybe add private: true to the package.json for now too.

[mbrookes]

@newoga - done.