Maven plugin doesn't handle extra sources

[tisonkun]

Testable repo - korandoru/hawkeye#99

hawkeye-gradle-plugin/src/functionalTest is an extra source directory, but the maven plugin cannot identify it or apply on it:

[DEBUG] Spotless will not format an up-to-date file: /Users/tison/Brittani/hawkeye/hawkeye-gradle-plugin/pom.xml
[INFO] Spotless.Format is keeping 1 files clean - 0 were changed to be clean, 0 were already clean, 1 were skipped because caching determined they were already clean
[DEBUG] Spotless will not format an up-to-date file: /Users/tison/Brittani/hawkeye/hawkeye-gradle-plugin/src/test/java/io/korandoru/hawkeye/gradle/plugin/HawkeyeGradlePluginTest.java
[DEBUG] Spotless will not format an up-to-date file: /Users/tison/Brittani/hawkeye/hawkeye-gradle-plugin/src/main/java/io/korandoru/hawkeye/gradle/plugin/HawkeyeGradlePlugin.java
[INFO] Spotless.Java is keeping 2 files clean - 0 were changed to be clean, 0 were already clean, 2 were skipped because caching determined they were already clean

Note that only files under src/main and src/test are processed.

[tisonkun]

If I use:

    <build>
        <sourceDirectory>src/functionalTest/java</sourceDirectory>
    </build>

It can help. But this will miss the src/main/java directory.

[basil]

Causes #1914.