Add a migrate package command

[MrTango]

To make it easier to migrate existing packages, to be compatible with plonecli/botemplates.plone, we need a migrate command. This command should update existing files and add needed placeholders.

1. adding placeholder line into all configure.zcml files
2. checking that either bobtemplates.cfg or pyproject.toml with plonecli/bobtemplate.plone settings exists, the later is the preferred solution for future package structure.

see https://bobtemplatesplone.readthedocs.io/upgrade-packages.html for details