Maven annotation processing processor not found

I've found the answer myself. I've figured out that the problem was the file javax.annotation.processing.Processor in META-INF/services/ with the configuration of the annotation processor's class. In order to fix the problem I had to add the following to the pom.xml configuration of my processor project:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <configuration>
                <compilerArgument>
                    -proc:none
                </compilerArgument>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugin>
    </plugins>
</build>

This let Maven build the classes into the actual jar and fixed the problem. I don't know if this is a bug or not but it surely looks strange to me. Thank you everybody for the help!

This is an extended version of the accepted answer above provided by @Aurasphere. Hopefully this will give some explanation to how the proposed solution works.

First, some background to what is happening here. Say, we want a custom annotation processor. We implement it and put it into a JAR as Maven artefact, so that it could be consumed by other projects. When such projects are being compiled, we want our annotation processor to be recognised by Java compiler and used appropriately. To make this happen, one needs to tell the compiler about a new custom processor. Compiler looks in the resources and checks FQN of classes listed in META-INF/services/javax.annotation.processing.Processor file. It tries to find these classes in classpath and load them to run the processing of annotations used upon classes that are currently being compiled.

So, we want our custom class to be mentioned in this file. We can ask a user of our library to put this file manually, but this is not intuitive and users could be frustrated why the promised processing of annotation doesn't work. That's why we might want to prepare this file in advance and deliver it together with the processor inside JAR of our Maven artefact.

The problem is that if we simply put this file with FQN of the custom processor in it, it will trigger compiler during compilation of our artefact, and since the processor itself is not yet compiled, the compiler will show the error about it. So we need to skip annotation processing to avoid this. This can be done using -proc:none, or with Maven:

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <proc>none</proc>
    </configuration>
</plugin>

We might have unit tests that will need our annotation processor. In Maven, test compilation is carried out after main sources are built, and all classes are already available including our processor. We just need to add special step during processing of test sources which would use our annotation processor. This can be done using:

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <executions>
        <execution>
            <id>process-test-annotations</id>
            <phase>generate-test-resources</phase>
            <goals>
                <goal>testCompile</goal>
            </goals>
            <configuration>
                <proc>only</proc>
                <annotationProcessors>
                    <annotationProcessor>fully.qualified.Name</annotationProcessor>
                </annotationProcessors>
            </configuration>
        </execution>
    </executions>
</plugin>