As an alternative, the attribute Automatic-Module-Name could be added (see https://docs.oracle.com/javase/9/docs/specs/jar/jar.html#Modular ). The primary advantage is that downstream users can use requires com.drew.imaging instead of requires metadata-extractor (which can change -- it is generally recommended to use reverse-dns for module names to avoid conflicts). This does not require xmpcore to do anything (although they should still add an Automatic-Module-Name to their metadata).

It does not allow some of the other advantages of the module system (like specifying what is actually usable outside of the module).

If a module-info.java file is desired, it is possible to compile everything once with Java 9+, then recompile everything except module-info.java with Java 6. Kind of a PITA, but it does help upstream developers hide internals from other people.

How are modules packaged and deployed? Does this still use Maven tooling and hosting?

Just a jar with an additional file (module-info.class). Maven has some documentation on it.

Sample patch that keeps Java 6 compatibility (although it needs to be compiled with Java 9+).
Notes:

You'll probably want to export specific classes, and the module name might be changed (e.g., com.drew.imaging.metadata-extractor)
requires xmpcore; will probably have to be updated at some time

diff --git a/Source/module-info.java b/Source/module-info.java
new file mode 100644
index 00000000..13650da2
--- /dev/null
+++ b/Source/module-info.java
@@ -0,0 +1,3 @@
+open module com.drew.imaging {
+    requires xmpcore;
+}
diff --git a/pom.xml b/pom.xml
index d940cce1..728279cf 100644
--- a/pom.xml
+++ b/pom.xml
@@ -117,11 +117,25 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
-                <version>3.8.1</version>
+                <version>3.10.1</version>
                 <configuration>
-                    <source>1.6</source>
-                    <target>1.6</target>
+                    <release>6</release>
                 </configuration>
+                <executions>
+                    <execution>
+                        <id>default-compile</id>
+                        <configuration>
+                            <release>9</release>
+                        </configuration>
+                    </execution>
+                    <execution>
+                        <id>base-compile</id>
+                        <goals><goal>compile</goal></goals>
+                        <configuration>
+                            <excludes><exclude>module-info.java</exclude></excludes>
+                        </configuration>
+                    </execution>
+                </executions>
             </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>

With that said, until xmp-core either is modularized or has declared an Automatic-Module-Name in their manifest, it is not recommended to fully modularize to avoid issues when xmp-core does modularize.

Possible source locations for xmp-core:

https://github.com/drewnoakes/adobe-xmp-core (doubtful)
https://git.corp.adobe.com/coretech/xmp-java/tree/mvncentral/core (looks like it is behind a corporate firewall or something)

Add module-info for java9+ #566

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