Consolidate packaging targets to handle multi-targeting & off-by-default source generators

[layomia]

Support for NuGet-packaging off-by-default source generators was added in #82179. Address feedback from @ViktorHofer in that PR.

cc @ericstj

Tagging subscribers to this area: @dotnet/area-extensions-configuration
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Support for NuGet-packaging off-by-default source generators was added in #82179. Address feedback from @ViktorHofer in that PR.

cc @ericstj

Author:	layomia
Assignees:	layomia
Labels:	packaging, area-Extensions-Configuration, source-generator
Milestone:	8.0.0

[layomia]

Rendered obsolete by #84379.