The package does not have assembly definitions, but you can add them with all required references.
Why no assembly definitions by default:
changes in the .asmdef files are lost with the package update
supported third-party packages do not use assembly definitions, so they cannot be referenced to be used in a separate assembly
version defines works only for Unity packages, so only direct references to assembly definitions are available
asmdef for the runtime should be created in the New UI Widgets folder
asmdef for the editor should be created in the New UI Widgets / Editor folder
InputSystemshould be added if you use them