Move modules into their own packages.

This sets the Automatic-Module-Name for moshi, moshi-adapters, and moshi-kotlin.
It moves moshi-adapters into its own .adapters package and forwards the existing
adapter. It moves the moshi-kotlin into its own .kotlin package and forwards the
existing adapter.

I'm not certain this is necessary or sufficient, but I think it's the right idea
for JPMS compatibility.

kotlin/pom.xml

@@ -84,6 +84,17 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-jar-plugin</artifactId>
+        <configuration>
+          <archive>
+            <manifestEntries>
+              <Automatic-Module-Name>com.squareup.moshi.kotlin</Automatic-Module-Name>
+            </manifestEntries>
+          </archive>
+        </configuration>
+      </plugin>
     </plugins>
   </build>
 </project>