contains com.google.common.util.concurrent.ListenableFuture class, without
    any other Guava classes. The idea is:

    - If users want only ListenableFuture, they depend on listenablefuture-1.0.

    - If users want all of Guava, they depend on guava, which, as of Guava
    27.0, depends on
    listenablefuture-9999.0-empty-to-avoid-conflict-with-guava. The 9999.0-...

          <offlineLinks>
            <!-- We need local copies of some of these for 2 reasons: a User-Agent problem (https://stackoverflow.com/a/47891403/28465) and an SSL problem (https://issues.apache.org/jira/browse/MJAVADOC-507). If we choose to work around the User-Agent problem, we can go back to <links>, sidestepping the SSL problem. -->

                <!-- TODO(cpovirk): Use .mvn/jvm.config instead (per
                     https://errorprone.info/docs/installation#maven) if it can
                     be made not to interfere with JDK8 or if we stop building
                     with JDK8. -->
                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>

          <offlineLinks>
            <!-- We need local copies of some of these for 2 reasons: a User-Agent problem (https://stackoverflow.com/a/47891403/28465) and an SSL problem (https://issues.apache.org/jira/browse/MJAVADOC-507). If we choose to work around the User-Agent problem, we can go back to <links>, sidestepping the SSL problem. -->

    without any of its other classes -- but is also available in a second
    "version" that omits the class to avoid conflicts with the copy in Guava
    itself. The idea is:

    - If users want only ListenableFuture, they depend on listenablefuture-1.0.

    - If users want all of Guava, they depend on guava, which, as of Guava
    27.0, depends on
    listenablefuture-9999.0-empty-to-avoid-conflict-with-guava. The 9999.0-...

            lines could make c.g.c.base.testModule transitively inherit from extra modules. If some
            of those modules are ones that it uses but forgets to list in its own <inherits>, we'd
            like to get an error. Currently we do, but if we add the extra <inherits> lines, we
            won't.

            I have one idea for a better approach, but it's painful, and I haven't tested it: We