Add the ddump-dir option/config value #4225

[waddlaw]

Fixes #4225

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

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• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

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[waddlaw]

@snoyberg this should be all set now :)
Is this alright? Could you please confirm it?

[snoyberg]

Looks like a great PR, just one minor comment.

[snoyberg]

It looks like a pattern match would be easier to read here, e.g.:

case T.unpack <$> boptsDdumpDir eebuildOpts of
  Just ddumpPath | buildingFinals && enableDdumpDir && not (null ddumpPath) -> do ...
  _ -> pure ()

[waddlaw]

That makes sense. That's true :)

[snoyberg]

LGTM. We can merge once CI passes.

[snoyberg]

Thanks!