`mgmt` generation invoked from `sphinx` tox env is generating sources for more than the targeted package

[scbedd]

Reported by @kristapratico , see here.

It appears that we're running doc generation for more than the targeted package. At the same time, we're definitely not actually generating docs, as that would take hours upon hours. Looks like we're uselessly generating sphinx sources, but only actually building docs for the target package.