Packages aliases in metainfo

We are currently moving a lot of metainfo packages. It is necessary to have some infrastructure that allows to gracefully deprecate old package names. The goal is to not have to reprocess or otherwise migrate data instantly.

"Package aliases" would allow to add old package names to a package definition:

• m_from_dict will consider package aliases when looking for definition referenced in m_def
• the gui will consider package aliases when looking for definitions referenced in m_def
• searches targeting section_defs will expand into an "or" on all package aliases
• facilities will be made available to clients to easily work with package names and consider package aliases in the process

Property aliases (espsecially for sub sections) is a related functionality, not covered in the issue.