re-use doctrine metadata to determine fields to map

_From @lsmith77 on October 28, 2017 19:27_

this will make it possible to add new properties to an entity and automatically supporting it via an importer without adjustments

portphp could be used here

relates to https://github.com/portphp/doctrine/pull/9

_Copied from original issue: solutionDrive/SyliusImportExportPlugin#8_