'implementation files(...)' does not put the jar on the module path

I have a modular project (“Enable module support” is checked in Eclipse) with

dependencies {
	implementation files("lib/modular.jar")

modular.jar is a modular jar (contains a module-info). The problem is that it’s not being put on the module path under Project and External Dependencies, so the module can’t be found in my own module-info.java in the requires com.module.from.jar; clause.
Is this a problem in Buildship or Gradle?

Eclipse 4.31
Buildship 3.1.9