Java 9 with Maven - modules not found

I had the same problem. What I finally did and worked was to insert the build-element from a pom created by Eclipse in my pom.xml:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.7.0</version>
            <executions>
                <execution>
                    <id>default-compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <source>9</source>
                        <target>9</target>
                    </configuration>
                </execution>
                <execution>
                    <id>default-testCompile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>testCompile</goal>
                    </goals>
                    <configuration>
                        <source>9</source>
                        <target>9</target>
                    </configuration>
                </execution>
            </executions>
            <configuration>
                <source>9</source>
                <target>9</target>
            </configuration>
        </plugin>
    </plugins>
</build>

I make this another answer because not everyone may read the comments...

For me the maven-compiler-plugin version was the problem. It seems modules are only supported since 3.8.0.

So here are the parts from my pom.xml:

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

    <maven.compiler.source>9</maven.compiler.source>
    <maven.compiler.target>9</maven.compiler.target>
</properties>

<build>
    <plugins>

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.1</version>
        </plugin>

    </plugins>
</build>