Fix Javadoc generation under JDK11.
"The code being documented uses modules but the packages defined in are in the unnamed module."
Compare to CL for Guava.
RELNOTES=n/a
<!-- TODO(cpovirk): Link to the version that we depend on? -->
<link>https://google.github.io/guava/releases/snapshot-jre/api/docs</link>
</links>
+ <source>8</source>
</configuration>
</plugin>
<plugin>
