Always query pypi for correct version

[tcompa]

• If the user provides no version for a fractal-task-core collection, we do query pypi to find the latest.
• If the user provides an incomplete information (e.g. version=1.2) for a fractal-task-core collection, we should also query pypi to transform it into a valid version.
• More generally, I'd say we should always query pypi before responding, for a pypi-based task collection.

The original observation was that sometimes we end up with invalid-semver versions in fractal-web, like 1.3 - cc @zonia3000

[tcompa]

Note that this is necessary, because of this scenario:

1. User collects fractal-tasks-core 1.3, which implicitly leads to 1.3.4.
2. User uses tasks with actual version 1.3.4, labeled as 1.3 - this is wrong.
3. Comparison with v1.3.2 says that 1.3 is older, but the actual v1.3.4 is more recent - this is wrong.
4. User collects fractal-tasks-core 1.3.4 and collection goes through - this is wrong.