Failing to add a dependency on a plugin available in the file system

I have 2 local plugins and I've been trying to set up one of them as a dependency for the other one. Following these instructions I encountered a problem.

build.gradle.kts:

intellij {
    version.set("2023.1.1")
    type.set("IC") // Target IDE Platform
    plugins.set(listOf("com.intellij.java"))
    // add my local plugin
    plugins.add(file("/home/jjnkn/fielden/git/idea-apt/build/idea-sandbox/plugins/idea-apt"))
}

Trying to reload Gradle configuration or to build the plugin fails with the following:

Could not find idea-apt-0.1.5-SNAPSHOT-0.1.5-SNAPSHOT-unzipped.com.jetbrains.plugins.jar (unzipped.com.jetbrains.plugins:fielden.idea.apt:0.1.5-SNAPSHOT).
Searched in the following locations:
    file:/home/jjnkn/.gradle/caches/modules-2/files-2.1/com.jetbrains.intellij.idea/ideaIC/2023.1.1/770a0552545fe6ab0de3e2f8ac9adb7ea3046417/ideaIC-2023.1.1/plugins/fielden.idea.apt/idea-apt-0.1.5-SNAPSHOT.jar
    file:/home/jjnkn/.gradle/caches/modules-2/files-2.1/com.jetbrains.intellij.idea/ideaIC/2023.1.1/770a0552545fe6ab0de3e2f8ac9adb7ea3046417/ideaIC-2023.1.1/idea-apt-0.1.5-SNAPSHOT.jar
    file:/home/jjnkn/.gradle/caches/modules-2/files-2.1/com.jetbrains.intellij.idea/unzipped.com.jetbrains.plugins/fielden.idea.apt-0.1.5-SNAPSHOT/idea-apt-0.1.5-SNAPSHOT.jar
    file:/home/jjnkn/.gradle/caches/modules-2/files-2.1/com.jetbrains.intellij.idea/ideaIC/2023.1.1/6ba1ff0f5cf440c16dd0c3b2f1fe5baacd53d918/idea-apt-0.1.5-SNAPSHOT-2023.1.1-unzipped.com.jetbrains.plugins.jar
    file:/home/jjnkn/fielden/git/idea-apt/build/idea-sandbox/plugins/idea-apt-0.1.5-SNAPSHOT.jar

Possible solution:
 - Declare repository providing the artifact, see the documentation at https://docs.gradle.org/current/userguide/declaring_repositories.html

Why would it search Gradle caches if the specified path already contains everything it needs?

Note that both plugins are configured with the same IDE version and type.

Contents of the specified path:

idea-apt
└── lib
   ├── checker-qual-3.33.0.jar
   ├── commons-lang3-3.0.jar
   ├── error_prone_annotations-2.18.0.jar
   ├── failureaccess-1.0.1.jar
   ├── guava-32.1.2-jre.jar
   ├── instrumented-idea-apt-0.1.5-SNAPSHOT.jar
   ├── javapoet-1.13.0.jar
   ├── jsr305-3.0.2.jar
   ├── listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
   ├── log4j-api-2.20.0.jar
   └── log4j-core-2.20.0.jar

Although the instructions say the following (which I have followed):

It is also possible to refer to the sandbox directory of another Gradle project — to do that, point to the /projects/plugin-name/build/idea-sandbox/plugins/plugin-name/ directory

I also tried specifying “/home/jjnkn/fielden/git/idea-apt/build/idea-sandbox/plugins/idea-apt/lib/” (the same path but with /lib appended), which also fails:

[gradle-intellij-plugin :] Cannot create plugin from file '/home/jjnkn/fielden/git/idea-apt/build/idea-sandbox/plugins/idea-apt/lib': Plugin descriptor 'plugin.xml' is not found

Next, I tried “/home/jjnkn/fielden/git/idea-apt/build/idea-sandbox/plugins/idea-apt/lib/idea-apt-0.1.5-SNAPSHOT.jar" after symlinking idea-apt-0.1.5-SNAPSHOT.jar to instrumented-idea-apt-0.1.5-SNAPSHOT.jar (it wasn't working with the instrumented- prefix). This almost worked. The sandbox instance is able to start but the specified plugin fails to load as its dependencies are missing:

Caused by: java.lang.ClassNotFoundException: org.apache.logging.log4j.LogManager PluginClassLoader(plugin=PluginDescriptor(name=Annotation Processing Tool, id=fielden.idea.apt, descriptorPath=plugin.xml, path=~/fielden/git/tg-IDE-plugins/intellij-idea/tg-idea-plugin/build/idea-sandbox/plugins/idea-apt-0.1.5-SNAPSHOT.jar, version=0.1.5-SNAPSHOT, package=null, isBundled=false), packagePrefix=null, instanceId=22, state=active)