How add local dependecy in assembly JAR - maven

Question

I have a multimodule Maven project with several dependencies. I want to build a fat executable JAR containing them as well as my own compiled classes. I found maven-assembly-plugin to be just what I needed except one nasty problem.
Some of my dependencies are local and distributed with project sources. I use system scope for them. It looks something like this:
<dependency>
<groupId>com.intellij</groupId>
<artifactId>forms_rt</artifactId>
<version>1</version>
<scope>system</scope>
<systemPath>${project.basedir}/lib/forms_rt.jar</systemPath>
</dependency>
The problem is that for some reason these libraries aren't unpacked and bundled with the rest of dependencies in result JAR.
I know that usage of system scope is considered bad practice, and in fact I even can find some of them (though quite outdated) in Maven repositories, but anyway it puzzles me how it can be solved with maven-assembly-plugin.
Just in case my plugin configuration looks like this:
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<archive>
<manifest>
<mainClass>repoll.server.Repoll</mainClass>
</manifest>
</archive>
<finalName>${project.build.finalName}-full</finalName>
<appendAssemblyId>false</appendAssemblyId>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
</execution>
</executions>
</plugin>
As I understood I have to write custom descriptor, which e.g. includes and unpacks all JARs from ${project.basedir}/lib directory, but after several unsuccessful attempts I still don't know how to do so.

Answer 1

I've managed to find solution here. It's described in this question.
In short, the whole problem was in usage of system scope. It turned out, that such dependencies are filtered out by default, which I found out by running mvn package with debug output enabled (-X/--debug).
When local repository is defined for these JARs, distributed with project, they are unpacked by maven-assembly-plugin as exepected.