[MNG-8713] SourceRoot.directory() default value should include the module when present (#2278)

The default directory is `src/${module}/${scope}/${lang}`.
When no module is defined, the previous default is unchanged.

Author: desruisseaux

api/maven-api-core/src/main/java/org/apache/maven/api/SourceRoot.java

@@ -39,11 +39,26 @@ public interface SourceRoot {
      * The path is relative to the <abbr>POM</abbr> file.
      *
      * <h4>Default implementation</h4>
-     * The default value is <code>src/{@linkplain #scope() scope}/{@linkplain #language() language}</code>
-     * as a relative path. Implementation classes may override this default with an absolute path instead.
+     * The default value depends on whether a {@linkplain #module() module name} is specified in this source root:
+     * <ul>
+     *   <li>
+     *     If no module name, then the default directory is
+     *     <code>src/{@linkplain #scope() scope}/{@linkplain #language() language}</code>.
+     *   </li><li>
+     *     If a module name is present, then the default directory is
+     *     <code>src/{@linkplain #module() module}/{@linkplain #scope() scope}/{@linkplain #language() language}</code>.
+     *   </li>
+     * </ul>
+     *
+     * The default value is relative.
+     * Implementation may override with absolute path instead.
      */
     default Path directory() {
-        return Path.of("src", scope().id(), language().id());
+        Path src = Path.of("src");
+        return module().map(src::resolve)
+                .orElse(src)
+                .resolve(scope().id())
+                .resolve(language().id());
     }
 
     /**

api/maven-api-model/src/main/mdo/maven.mdo

@@ -2014,9 +2014,15 @@
         usage (main code, tests, <i>etc.</i>) is specified by the {@code scope} element.
 
         <h2>Default source directories</h2>
-        If no source directories are specified, the defaults are {@code src/${scope}/${lang}} where
-        {@code ${scope}} is the value of the {@link #scope} element (typically {@code main} or {@code test}) and
-        {@code ${lang}} is the value of the {@link #lang} element (typically {@code java} or {@code resources}).
+        If no source directories are specified, the default values depend on whether module names are specified:
+        <ul>
+          <li>{@code src/${scope}/${lang}} if no module is specified</li>
+          <li>{@code src/${module}/${scope}/${lang}} if a module is specified</li>
+        </ul>
+        where
+        {@code ${scope}} is the value of the {@link #scope} element (typically {@code main} or {@code test}),
+        {@code ${lang}} is the value of the {@link #lang} element (typically {@code java} or {@code resources}),
+        and {@code ${module}} is the optional {@link #module} element.
         ]]>
       </description>
       <version>4.1.0+</version>

impl/maven-impl/src/main/java/org/apache/maven/impl/DefaultSourceRoot.java

@@ -81,7 +81,11 @@ public DefaultSourceRoot(final Session session, final Path baseDir, final Source
         if (value != null) {
             directory = baseDir.resolve(value);
         } else {
-            directory = baseDir.resolve("src").resolve(scope.id()).resolve(language.id());
+            Path src = baseDir.resolve("src");
+            if (moduleName != null) {
+                src = src.resolve(language.id());
+            }
+            directory = src.resolve(scope.id()).resolve(language.id());
         }
 
         value = nonBlank(source.getTargetVersion());