Add skip flags

This introduces 3 flags:
- `flatten.skip`: skips the entire plugin regardless of goal
- `flatten.clean.skip`: skips the clean goal
- `flatten.flatten.skip`: skips the flatten goal

This means the user has control over what parts are skipped, and the
flags are descriptive. The `flatten.` prefix is in line with other
parameters like `flatten.mode` and `flatten.dependency.mode`.

src/main/java/org/codehaus/mojo/flatten/AbstractFlattenMojo.java

@@ -43,6 +43,12 @@ public abstract class AbstractFlattenMojo extends AbstractMojo {
     @Parameter(property = "flattenedPomFilename", defaultValue = ".flattened-pom.xml")
     private String flattenedPomFilename;
 
+    /**
+     * If {@code true} the plugin will be skipped.
+     */
+    @Parameter(property = "flatten.skip", defaultValue = "false")
+    private boolean skip;
+
     /**
      * The constructor.
      */
@@ -71,4 +77,13 @@ public File getOutputDirectory() {
     protected File getFlattenedPomFile() {
         return new File(getOutputDirectory(), getFlattenedPomFilename());
     }
+
+    protected boolean shouldSkip() {
+        if (skip) {
+            return true;
+        }
+        return shouldSkipGoal();
+    }
+
+    protected abstract boolean shouldSkipGoal();
 }

src/main/java/org/codehaus/mojo/flatten/CleanMojo.java

@@ -24,6 +24,7 @@
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugin.MojoFailureException;
 import org.apache.maven.plugins.annotations.Mojo;
+import org.apache.maven.plugins.annotations.Parameter;
 
 /**
  * This MOJO realizes the goal <code>flatten:clean</code> that deletes any files created by
@@ -42,6 +43,12 @@
         threadSafe = true)
 public class CleanMojo extends AbstractFlattenMojo {
 
+    /**
+     * If {@code true} the clean goal will be skipped.
+     */
+    @Parameter(property = "flatten.clean.skip", defaultValue = "false")
+    private boolean skipClean;
+
     /**
      * The constructor.
      */
@@ -53,6 +60,10 @@ public CleanMojo() {
      * {@inheritDoc}
      */
     public void execute() throws MojoExecutionException, MojoFailureException {
+        if (shouldSkip()) {
+            getLog().info("Clean skipped.");
+            return;
+        }
 
         File flattenedPomFile = getFlattenedPomFile();
         if (flattenedPomFile.isFile()) {
@@ -63,4 +74,9 @@ public void execute() throws MojoExecutionException, MojoFailureException {
             }
         }
     }
+
+    @Override
+    protected boolean shouldSkipGoal() {
+        return skipClean;
+    }
 }

src/main/java/org/codehaus/mojo/flatten/FlattenMojo.java

@@ -364,6 +364,12 @@ public class FlattenMojo extends AbstractFlattenMojo {
     @Parameter(property = "flatten.dependency.keepComments", required = false, defaultValue = "false")
     private boolean keepCommentsInPom;
 
+    /**
+     * If {@code true} the flatten goal will be skipped.
+     */
+    @Parameter(property = "flatten.flatten.skip", defaultValue = "false")
+    private boolean skipFlatten;
+
     @Inject
     private DirectDependenciesInheritanceAssembler inheritanceAssembler;
 
@@ -400,6 +406,10 @@ public FlattenMojo() {
      */
     @Override
     public void execute() throws MojoExecutionException, MojoFailureException {
+        if (shouldSkip()) {
+            getLog().info("Flatten skipped.");
+            return;
+        }
 
         getLog().info("Generating flattened POM of project " + this.project.getId() + "...");
 
@@ -422,6 +432,11 @@ public void execute() throws MojoExecutionException, MojoFailureException {
         }
     }
 
+    @Override
+    protected boolean shouldSkipGoal() {
+        return skipFlatten;
+    }
+
     /**
      * This method extracts the XML header comment if available.
      *