So, before using .asmdef files, all your code would get compiled into a single assembly (well - 1 for runtime and 1 for editor code).
With this setup now, anything under Assets/Scripts will get compiled into 1 assembly, and everything under Assets/Tests will be compiled into another assembly.
These assemblies do not automatically reference any other code, and since your code under Assets/Plugins is not part of those assemblies, and it is not references by anything, you get this error.
To solve this, you can set up your Assets/Plugins under its own assembly definition and have your main assembly reference that.
Ok, I did as you said, and the errors are just multiplying…
Say, if in Assets/Plugins I have several folders each with a different plugin, do I have to put an asmdef on each folder or can I just put an asmdef in Assets/Plugins that automatically compiles every plugin? I did the second, and the errors persist.