Refactored picked versions logic to make it easier to use for plugins
This includes renaming `store_picked_versions` to `store_required_by`
to better signify its intent and adding `get_picked_versions`
to `zc.buidlout.easy_install`.
Added tests (and made them pass) for update_versions_file's output.
Merge branch 'refactor-pickedversions' of https://github.com/jaap3/buildout into jaap3-refactor-pickedversions
Changelog for PR #88.
Merge branches 'jaap3-refactor-pickedversions' and 'master'