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 76.91%. Comparing base (f5b1e5a) to head (466a71c).
⚠️ Report is 77 commits behind head on develop.

❌ Your project check has failed because the head coverage (76.91%) 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 (f5b1e5a) and HEAD (466a71c). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (f5b1e5a)	HEAD (466a71c)
	8	3

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #862      +/-   ##
===========================================
- Coverage    83.91%   76.91%   -7.00%     
===========================================
  Files           36       36              
  Lines         4277     4272       -5     
  Branches       531      528       -3     
===========================================
- Hits          3589     3286     -303     
- Misses         494      791     +297     
- Partials       194      195       +1     

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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.

[pjonsson]

Still doesn't work with the fixes in Click 8.3.2, but Click 8.3.3 should contain a fix that might unblock this.