Main class is not being set by maven-jmod-plugin

Hi!,

I have created a project in and [https://github.com/andretadeu/maven-jigsaw-examples/tree/master/multi-modules-jmod-jlink|multi-modules-jmod-jlink] and I needed to set the parameter --main-class in jmod create command.

Trying to add the <mainClass> tag:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jmod-plugin</artifactId>
    <configuration>
        <mainClass>com.andretadeu.greetings.Main</mainClass> <!-- This-->
        <modulePath>target/jmods</modulePath>
        <moduleVersion>${project.version}</moduleVersion>
    </configuration>

Maven gave the following error during the execution of maven-jlink-plugin:

[ERROR]
[ERROR] com.andretadeu.greetings file:///home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings/target/jmods/com.andretadeu.greetings.jmod
[ERROR] com.andretadeu.world file:///home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/world/target/jmods/com.andretadeu.world.jmod
[ERROR] java.base file:///home/andre/.sdkman/candidates/java/11.0.1-open/jmods/java.base.jmod
[ERROR]
[ERROR] Providers:
[ERROR] java.base provides java.nio.file.spi.FileSystemProvider used by java.base
[ERROR] Error: java.lang.IllegalArgumentException: com.andretadeu.greetings doesn't contain main class & main not specified in command line
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for multi-modules-jmod-jlink 1.0-SNAPSHOT:
[INFO]
[INFO] multi-modules-jmod-jlink ........................... SUCCESS [ 0.185 s]
[INFO] com.andretadeu.world ............................... SUCCESS [ 3.974 s]
[INFO] com.andretadeu.greetings ........................... SUCCESS [ 0.628 s]
[INFO] greetings-jlink .................................... FAILURE [ 3.068 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 8.916 s
[INFO] Finished at: 2018-12-11T22:42:31-02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-jlink-plugin:3.0.0-alpha-2-SNAPSHOT:jlink (default-jlink) on project greetings-jlink:
[ERROR] Exit code: 1
[ERROR] Command line was: /bin/sh -c /home/andre/.sdkman/candidates/java/11.0.1-open/bin/jlink @/home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings-jlink/target/jlinkArgs
[ERROR]
[ERROR]
[ERROR] -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-jlink-plugin:3.0.0-alpha-2-SNAPSHOT:jlink (default-jlink) on project greetings-jlink:
Exit code: 1
Command line was: /bin/sh -c /home/andre/.sdkman/candidates/java/11.0.1-open/bin/jlink @/home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings-jlink/target/jlinkArgs

I noticed that the --main-class parameter was not set in **jmodCreateArgs:

create
--module-version
1.0-SNAPSHOT
--class-path
/home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings/target/classes
--module-path
"/home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings/target/classes:/home/andre/.sdkman/candidates/java/11.0.1-open/jmods"
/home/andre/workspaces/oss/maven-jigsaw-examples/multi-modules-jmod-jlink/greetings/target/jmods/com.andretadeu.greetings.jmod

Debugging the plugin, I have found the root cause of the problem and I will make a pull request after publishing this issue.

Thanks,

André Tadeu de Carvalho