Maven plugin for dumping dependencies (cross-repo navigation)

[antonsviridov-src]

GRAPH-796

This PR adds a maven plugin that can be used to produce dependencies.txt file necessary for cross-repo navigation on Sourcegraph.

It can be used in a standalone invocation:

$ mvn com.sourcegraph:maven-plugin:<VERSION>:sourcegraphDependencies

Or added directly to maven build and run with sourcegraph:sourcegraphDependencies

  <plugin>
      <groupId>com.sourcegraph</groupId>
      <artifactId>maven-plugin</artifactId>
      <version>${scip-java.version}</version>
      <executions>
          <execution>
              <goals>
                  <goal>sourcegraphDependencies</goal>
              </goals>
          </execution>
      </executions>
  </plugin>

Test plan

• New maven project is added, and a new CI workflow that indexes it using the plugin

[antonsviridov-src]

Maven has a special packaging mode for plugins, which seems to be nothing more than a post-processing step that introspects the plugin's pom.xml + all the Mojo files, then writes the special plugin.xml file.

https://maven.apache.org/guides/plugin/guide-java-plugin-development.html

Setting up a Maven build just for that seems to be counterproductive, so we just write the plugin.xml by hand using a template. It's obviously not great, but at this point I don't see a better solution, and we don't expect this plugin to change very frequently.

plugin.xml will be force-kept in sync by tests, because Maven derives a lot of information from it, so if it's broken - things won't work.

For local development this allows you to do mavenPlugin/publishM2 and use that in maven builds.

[olafurpg]

Makes sense. I love sbt

[olafurpg]

LGTM 👍🏻 Very nice to finally have a Maven plugin. After using Gradle more, I've become fonder of Maven for Java projects (although sbt is still the best JVM build tool, haven't tried Mill much)

[olafurpg]

Makes sense. I love sbt

[olafurpg]

TSV ftw!