Move import to be dask-dist friendly

[gerritholl]

Make trollflow2 more multiprocessing / dask-distributed friendly. Move the launcher import in satpy_launcher.py within the main function, so that it only gets executed in the primary process. The call to Manager() in trollflow2.__init__.py was causing attempts to use trollflow2 with dask distributed to fail with a RuntimeError.

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Tests passed
• [x] Passes flake8 trollflow2
• [ ] Fully documented

I don't know how to add a unit test for this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.78%. Comparing base (c337b9e) to head (cb4cab3). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #208 +/- ## ======================================= Coverage 95.78% 95.78% ======================================= Files 12 12 Lines 2989 2989 ======================================= Hits 2863 2863 Misses 126 126 ``` | [Flag](https://app.codecov.io/gh/pytroll/trollflow2/pull/208/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/trollflow2/pull/208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `95.78% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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