Allow <includePluginDependencies> to be specified for the exec:exec goal

[adamretter]

This allows Java tools which are external to the dependencies of the project itself to be run using exec:exec.

For example, if I want to run The JAXB compiler xjc during the build process, I don't need it as an actual dependency of my project at runtime, but I do need it as a dependency of the exec plugin.

For example:

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>exec-maven-plugin</artifactId>
    <version>3.0.1-SNAPSHOT</version>
    <executions>
        <execution>
            <id>generate-jaxb-classes</id>
            <phase>generate-sources</phase>
            <goals>
                <goal>exec</goal>
            </goals>
            <configuration>
                <includePluginDependencies>true</includePluginDependencies>
                <executable>java</executable>
                <arguments>
                    <argument>-classpath</argument>
                    <classpath/>
                    <argument>com.sun.tools.xjc.XJCFacade</argument>
                    <argument>-verbose</argument>
                    <argument>-d</argument>
                    <argument>${project.build.directory}/generated-sources/jaxb</argument>
                    <argument>-b</argument>
                    <argument>${project.basedir}/src/main/xjb</argument>
                    <argument>${project.basedir}/src/main/xsd</argument>
                </arguments>
            </configuration>
        </execution>
    </executions>
    <dependencies>
        <dependency>
            <groupId>com.sun.xml.bind</groupId>
            <artifactId>jaxb-xjc</artifactId>
            <version>3.0.0</version>
        </dependency>
    </dependencies>
</plugin>

Before this PR the above configuration would fail with:

Error: Could not find or load main class com.sun.tools.xjc.XJCFacade

[ERROR] Command execution failed.
org.apache.commons.exec.ExecuteException: Process exited with an error: 1 (Exit value: 1)
    at org.apache.commons.exec.DefaultExecutor.executeInternal (DefaultExecutor.java:404)
    at org.apache.commons.exec.DefaultExecutor.execute (DefaultExecutor.java:166)
    at org.codehaus.mojo.exec.ExecMojo.executeCommandLine (ExecMojo.java:1002)
...

[adamretter]

@olamy @khmarbaise Any chance of getting some review on this? I included all the good stuff (tests, passing CI) ;-)

[dizzzz]

Good stuff @adamretter , very useful

[adamretter]

@olamy @khmarbaise Just a gentle ping to see if there is any chance of getting this merged?

[olamy]

sounds very useful. would you mind resolve conflicts?

[slawekjaranowski]

suppressed by #432