Reenable relative imports in Module Repository

This MR fixes #81, where importing an enum from two different modules resulted in seperate instances of the same definition.

Description

Setup:
To check this, this MR adds a unit test, where an enum is imported (relative import) by another module.

Problem:

• Previously, this lead to errors. Reason for this behavior is that each module is interpreted gets "sandboxed" into a seperate parent-module with a random id. Using a relative import circumvents this mechanism.
• On the contrary, sandboxing modules is required when handling either with multiple miniflask-instances (as done for unit-testing) or modules of the same name in different module repositories on the filesystem. In both cases, we have to handle how to distinguish modules with the same local id.

New Behavior:

• The solution is to sandbox each miniflask-instance instead of each module.
• This enables relative imports in between modules and repositories, using the repository-ids given upon repository registration (i.e. using dict-registration).
• The new behavior requires __init__.py files to be present for the whole relative-import path. This is in comply with typical relative imports in python.

Check all before creating this PR:

• unit tests adapted / created