Cleanup task-collection modules and tests

[tcompa]

The tasks/collection.py module is quite messy, and its tests are scattered in different modules with no obvious logic. This is confusing, while developing e.g. PR #1188.

Within tasks, we should separate e.g.:

1. Schemas or any payload validation logic;
2. Functions that are used within the task-collection endpoint (including preliminary tests - ref Prevent task-package name conflict #1189);
3. Functions that are executed as a background task.

A similar organization can be used for unit/API tests.

This will help us to clearly determine which functions rely on which assumptions (e.g. which functions use normalized package names).

[tcompa]

I will close this with #1188, which includes a refactor that does not change functionality (on top of the actual changes related to #736). The new structure is as follows

$ tree fractal_server/tasks/
fractal_server/tasks/
├── background_operations.py
├── endpoint_operations.py
├── __init__.py
├── _TaskCollectPip.py
└── utils.py

0 directories, 5 files

$ tree tests/07_task_collection/
tests/07_task_collection/
├── test_background_operations.py
├── test_endpoint_operations.py
└── test_unit_tasks.py

0 directories, 3 files

which at least places definitions in some appropriately scoped module.

Note that a review of the task-collection flow is still due, although not urgent. As an example, to start with, the fact that we often call the check method is a hint that some abstractions are not correct.