Wrong "Package not found" warning

FULL PRODUCT VERSION :
openjdk version "9.0.1"
OpenJDK Runtime Environment (build 9.0.1+11)
OpenJDK 64-Bit Server VM (build 9.0.1+11, mixed mode)

java version "10-ea"
Java(TM) SE Runtime Environment (build 10-ea+29)
Java HotSpot(TM) 64-Bit Server VM (build 10-ea+29, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux 4.4.0-98-generic #121-Ubuntu SMP Tue Oct 10 14:24:03 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
We have a top-level package "org.openstreetmap.josm.spi" containing only a package-info.java (no class). Then we have a sub-package "org.openstreetmap.josm.spi.preferences" containing the classes.

When running javadoc tool from Ant, we get this warnings with JDK 9, 9.01, and 10-ea+b29:

"Package org.openstreetmap.josm.spi not found"

We don't understand why, as there is no such warning with JDK8 (tested OK with JDK8u152 and JDK8u162-ea-b01).

As per Quality Outreach, please add label "josm-found" to this bug report.

REGRESSION. Last worked in version 8u162

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
svn co https://josm.openstreetmap.de/svn/trunk
ant javadoc | grep -i warn

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No warnings
ACTUAL -
  [javadoc] Constructing Javadoc information...
  [javadoc] javadoc: warning - Package org.openstreetmap.josm.spi not found
  [javadoc] Standard Doclet version 9.0.1

ERROR MESSAGES/STACK TRACES THAT OCCUR :
javadoc: warning - Package org.openstreetmap.josm.spi not found

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
Here is the single file of the package (package-info.java):

// License: GPL. For details, see LICENSE file.

/**
 * This package contains services.
 * <ul>
 * <li>Abstract service interfaces and
 * <li>manager classes to install a service provider and give access to the
 * functionality of the service.
 * </ul>
 * Client code will only depend on the classes from the spi package, but not on
 * implementations of the service interface (service providers) which are found
 * elsewhere.
 * <p>
 * The concept is similar to Java Service Provider Interfaces (hence the name),
 * except the service providers are registered directly with a method call instead
 * of using {@link java.util.ServiceLoader} to discover the providers on classpath.
 */
package org.openstreetmap.josm.spi;

---------- END SOURCE ----------