Packaged plugins that are added to projects that were made with an Engine built from source code will indicate that the plugin .dll needs to be built. If the plugin was created and packaged using a binary Engine, and the packaged plugin is then added to a different project also created in the same version of the binary Engine, the project will open normally.
4.20 Bug Scrub
Setup:
<Engine_Workspace>\Engine\Build\BatchFiles\RunUAT.bat BuildPlugin -Plugin=<path_to_plugin> -Package=<path_to_output_folder>
Original
RESULT:
A notice appears that the plugin .dll needs to be built.
EXPECTED:
The project opens in the Editor.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45855 in the post.