guix import pypi fails if one of the transitive deps has no source on PyPi

[jgart]

I realize this may be a feature some may want but then we get no template
to work from because `guix import pypi` is to heavy handed:

$ guix import pypi neoscore -r

Starting download of /tmp/guix-file.HrGkox
>From 
>https://files.pythonhosted.org/packages/f8/41/97964d689a5f7e10792ffd13c98d6348368bbf93fd473b4da135fbfc9412/neoscore-0.1.7.tar.gz...
 ….1.7.tar.gz  869KiB                 3.5MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.vc9mqy
>From 
>https://files.pythonhosted.org/packages/0c/28/8830d1da820115c4f342ce48fa337a7a77fd4e04a6ef68bf4cd17e5e6796/neoscore-0.1.7-py3-none-any.whl...
 …py3-none-any.whl  919KiB            3.7MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.pIjiPU
>From 
>https://files.pythonhosted.org/packages/e1/57/2023316578646e1adab903caab714708422f83a57f97eb34a5d13510f4e1/PyQt5-5.15.7.tar.gz...
 …5.7.tar.gz  3.1MiB                  6.9MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.p5Hob2
>From 
>https://files.pythonhosted.org/packages/14/75/596d5e9ed7a135918bb157ed315004ac008e09b9b4c9328f94568c88f003/PyQt5-5.15.7-cp37-abi3-macosx_10_13_x86_64.whl...
 …macosx_10_13_x86_64.whl  6.7MiB     8.7MiB/s 00:01 [##################] 100.0%

Starting download of /tmp/guix-file.1vSRMl
>From 
>https://files.pythonhosted.org/packages/08/83/4cea5c665d2af765c02f7d8e8560b5918405c1d7d11ccfc60c4919c1cfd0/backports.cached-property-1.0.2.tar.gz...
 …operty-1.0.2.tar.gz  8KiB           3.2MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.r5HeLH
>From 
>https://files.pythonhosted.org/packages/eb/ae/69e52acdcf381b108b36d989ea58656de4a9ab8863aba6176d80d01041df/backports.cached_property-1.0.2-py3-none-any.whl...
 …y-1.0.2-py3-none-any.whl  6KiB      4.3MiB/s 00:00 [##################] 100.0%

Starting download of /tmp/guix-file.J00iQN
>From 
>https://files.pythonhosted.org/packages/95/b5/f933f482a811fb9a7b3707f60e28f2925fed84726e5a6283ba07fdd54f49/img2pdf-0.4.4.tar.gz...
 …4.4.tar.gz  96KiB                   1.3MiB/s 00:00 [##################] 100.0%
guix import: error: no source release for pypi package PyQt5-Qt5 5.15.2

hint: This indicates that the package is available on PyPI, but only as a 
"wheel" containing binaries, not
source.  To build it from source, refer to the upstream repository at
`https://www.riverbankcomputing.com/software/pyqt/'.

Can we use the maybe monads here to prevent this?