Description of problem:
It seems like the "download" plugin needs to be adopted for the debuginfo/debugsource split. It has been introduced in Fedora 27 
Version-Release number of selected component (if applicable):
Steps to Reproduce:
Try to download the debugsource package by running either
1. $ dnf download --debuginfo nspr-debugsource
2. $ dnf download --debugsource nspr-debugsource
The first command downloads the debuginfo-file (containing no debug source) although I specifically requested otherwise.
The second command fails with this error message:
> dnf download: error: unrecognized arguments: --debugsource
Download debugsource package.
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able to fix it before Fedora 29 is end of life. If you would still like
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version prior this bug is closed as described in the policy above.
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I am closing this as NEXTRELEASE because the patch requires newer version of dnf than is available in f29.
It is fixed in dnf-plugins-core-4.0.11-1.fc30 (>= fedora 30)
If this is really needed in f29 please let us know we could also back-port the necessary changes to dnf along with the patch to dnf-plugins-core.