Add module-info.java for JPMS support using multi-release JAR

[Copilot]

Adds Java Platform Module System (JPMS) support via module-info.java while maintaining Java 8 compatibility using a multi-release JAR approach.

Changes

• Created src/main/java9/module-info.java: Declares module org.codehaus.plexus.interpolation with exports for all 7 public packages (interpolation, fixed, multi, object, os, reflection, util)

• Updated pom.xml: Added a java9+ Maven profile that activates on JDK 9+ using multi-release JAR (MRJAR) approach:

  • Profile activates automatically when building with JDK 9 or higher via <jdk>[9,)</jdk>
  • compile-java9: Compiles module-info.java from src/main/java9/ with Java 9 and places it in META-INF/versions/9/
  • maven-jar-plugin: Configured to add Multi-Release: true to MANIFEST.MF
  • When building with JDK 8, the profile is inactive and module-info.java is skipped
  • No JDK toolchain configuration required

Result: When built with JDK 9+, a multi-release JAR is created with module-info.class in META-INF/versions/9/ (bytecode version 53) and all other classes remain Java 8 (bytecode version 52). When built with JDK 8, only Java 8 classes are produced. OSGi Bundle metadata in MANIFEST.MF unchanged.

module org.codehaus.plexus.interpolation {
    exports org.codehaus.plexus.interpolation;
}

• Fixes Add/generate module descriptor #89

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add/generate module descriptor</issue_title>
<issue_description>this project has no dependencies and the manifest already contains Bundle info, so it should be a small step to also have the module-info.java</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add/generate module descriptor #89

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[slawekjaranowski]

@rfscholte @slachiewicz do we need export all packages ....?

eg org.codehaus.plexus.interpolation.os look like internal one?

[rfscholte]

my hope was only org.codehaus.plexus.interpolation;

however, a quick scan through the Maven codebase:
DefaultToolchainsBuilderTest uses org.codehaus.plexus.interpolation.os.OperatingSystemUtils
PathTranslatingPostProcessor uses org.codehaus.plexus.interpolation.util.ValueSourceUtils

as long as Maven is not using the modulepath that's not an issue.

I would probably go for only org.codehaus.plexus.interpolation and investigate if there are requests for opening more packages