generate: always use Pool

[pjonsson]

The imap implementation will be
as lazy as one could expect in
Python 3.15, but until then
it is more eager than one would
expect.

Switch from imap_unordered to
using apply_async instead, but
print the results in a deterministic
order and remove the special
case where a different code path
is taken when running on a single
core.

---

📚 Documentation preview 📚: https://datacube-explorer--862.org.readthedocs.build/en/862/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.06%. Comparing base (27e8564) to head (2a3179b).
⚠️ Report is 1 commits behind head on develop.

❌ Your project check has failed because the head coverage (77.06%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (27e8564) and HEAD (2a3179b). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (27e8564)	HEAD (2a3179b)
	4	3

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #862      +/-   ##
===========================================
- Coverage    84.23%   77.06%   -7.18%     
===========================================
  Files           35       35              
  Lines         4199     4194       -5     
  Branches       526      523       -3     
===========================================
- Hits          3537     3232     -305     
- Misses         471      768     +297     
- Partials       191      194       +3     

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[pjonsson]

This is failing because the Click CLI runner does not flush stderr as it should until Click 8.2.1, so the test output is not appearing as expected.

Unfortunately Explorer cannot use Click 8.2.1 because that changed something else causing multiple test failures, but I believe Click 8.3.0 will fix that issue.

Putting this as a draft for now until Click 8.3.0 is released.