(docs) Add moditect-maven-plugin as it is a less complex way for Java 8 with java module descriptors

[bmarwell]

Adds the less complex moditect plugin to the module-info docs which is probably more helpful than compiling sources twice.

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[desruisseaux]

Actually the section saying that we need to run javac twice could be updated. It should not be needed any more since the addition of the multi-release support, although we would need to test for making sure. But in principle, a configuration like below should work with Java 8, with the module-info.java file in the src/main/java_9 directory:

    <sources>
      <source>
        <directory>src/main/java</directory>
        <targetVersion>8</targetVersion>
      </source>
      <source>
        <directory>src/main/java_9</directory>
        <targetVersion>9</targetVersion>
      </source>
    </sources>

We could amend the pull request so that instead of being "two compilations versus one compilation", make it "explicit module-info versus module-info derived from the pom.xml". We may need to test the above snippet first for making sure that it works in this context too.

[desruisseaux]

I tried the approach described in above comment and it did not worked. It is a bit tricky to compile only a module-info.java file when everything else has been compiled as a class-path project. I just noticed that indeed, the existing text about using maven-compiler-plugin compiles the whole project twice. Therefore, I have no change to propose.