Split metador-core into multiple packages to avoid possibly unneeded dependencies for users

[apirogov]

When the interfaces stabilize, this package should be split up so its not required to pull a huge pile of dependencies to use a fraction of the features.

The core should only provide the minimum:

• The plugin system that allows to define and load other plugin groups
• MetadorContainer interface, it is just dependent on schemas (and optional HDF5)
• The schema and harvester plugin group, packer plugin group, example packer
• possibly generic utilities (types, helper functions, etc)
• all the default schemas everybody is supposed to use (ROCrate compatible etc.)
• all the default harvesters (but their dependencies should be optional)

At this point a ZIP backend should exist, based on the python default zipfile.

Widgets clearly should move out completely, because of the whole flask/panel related dependencies:

• metador-widgets: Widgets plugin group, default widgets (only needs schemas + container), generic dashboard and its schema