Verify support for multiple main source folders (#1031)

* Add support for multiple main source folders

Similar to the support for additional test source folders added in PR #888,
this change enables the rewrite-maven-plugin to process multiple main source
directories configured via build-helper-maven-plugin or similar tools.

Changes:
- Modified MavenMojoProjectParser to iterate through all compile source roots
  using mavenProject.getCompileSourceRoots() instead of single source directory
- Added integration test to verify recipes are applied to files in all main
  source directories
- Follows the same pattern as the existing test source folder implementation

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Co-Authored-By: Claude <noreply@anthropic.com>

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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people

src/test/java/org/openrewrite/maven/RewriteRunIT.java

@@ -56,6 +56,20 @@ void multi_source_sets_project(MavenExecutionResult result) {
           .contains("Changes have been made to %s by:".formatted(separatorsToSystem("project/src/test/java/sample/RegularTest.java")));
     }
 
+    @MavenGoal("generate-sources")
+    @MavenTest
+    @SystemProperties({
+      @SystemProperty(value = "rewrite.activeRecipes", content = "org.openrewrite.java.format.SingleLineComments"),
+    })
+    void multi_main_source_sets_project(MavenExecutionResult result) {
+        assertThat(result)
+          .isSuccessful()
+          .out()
+          .warn()
+          .contains("Changes have been made to project/src/main/java/sample/MainClass.java by:")
+          .contains("Changes have been made to project/src/additional-main/java/sample/AdditionalMainClass.java by:");
+    }
+
     @MavenTest
     void single_project(MavenExecutionResult result) {
         assertThat(result)

src/test/resources-its/org/openrewrite/maven/RewriteRunIT/multi_main_source_sets_project/pom.xml

@@ -0,0 +1,40 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.openrewrite.maven</groupId>
+    <artifactId>multi_main_source_sets_project</artifactId>
+    <version>1.0</version>
+    <packaging>jar</packaging>
+
+    <properties>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>build-helper-maven-plugin</artifactId>
+                <version>3.6.0</version>
+                <executions>
+                    <execution>
+                        <id>add additional main sources</id>
+                        <phase>generate-sources</phase>
+                        <goals>
+                            <goal>add-source</goal>
+                        </goals>
+                        <configuration>
+                            <sources>
+                                <source>src/additional-main/java</source>
+                            </sources>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

src/test/resources-its/org/openrewrite/maven/RewriteRunIT/multi_main_source_sets_project/src/additional-main/java/sample/AdditionalMainClass.java

@@ -0,0 +1,8 @@
+package sample;
+
+public class AdditionalMainClass {
+    //This is a test class
+    public void method() {
+        System.out.println("Additional main source set");
+    }
+}

src/test/resources-its/org/openrewrite/maven/RewriteRunIT/multi_main_source_sets_project/src/main/java/sample/MainClass.java

@@ -0,0 +1,8 @@
+package sample;
+
+public class MainClass {
+    //This is a test class
+    public void method() {
+        System.out.println("Main source set");
+    }
+}