Document that repackage should not be used on the command-line

This commit clarifies how we intend the spring-boot:repackage goal to
be used. As it operates on the source jar (or war) that must be
effectively up-to-date to produce an accurate result, the package phase
must have run.

Contrary to build-image that was designed to be used on the command-line
by forking a package lifecycle first, repackage does not do that. This
commit also clarifies that by providing a more focused error message.

Closes gh-22317

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/antora/modules/maven-plugin/pages/packaging.adoc

@@ -10,6 +10,10 @@ Packaging an executable archive is performed by the `repackage` goal, as shown i
 include::example$packaging/repackage-pom.xml[tags=repackage]
 ----
 
+WARNING: The `repackage` goal is not meant to be used alone on the command-line as it operates on the source
+`jar` (or `war`) produced by the `package` phase.
+To use this goal on the command-line, you must include the `package` phase: `mvn package spring-boot:repackage`.
+
 TIP: If you are using `spring-boot-starter-parent`, such execution is already pre-configured with a `repackage` execution ID so that only the plugin definition should be added.
 
 The example above repackages a `jar` or `war` archive that is built during the package phase of the Maven lifecycle, including any `provided` dependencies that are defined in the project.

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

@@ -221,6 +221,10 @@ public class RepackageMojo extends AbstractPackagerMojo {
 	private void repackage() throws MojoExecutionException {
 		Artifact source = getSourceArtifact(this.classifier);
 		File target = getTargetFile(this.finalName, this.classifier, this.outputDirectory);
+		if (source.getFile() == null) {
+			throw new MojoExecutionException(
+					"Source file is not available, make sure 'package' runs as part of the same lifecycle");
+		}
 		Repackager repackager = getRepackager(source.getFile());
 		Libraries libraries = getLibraries(this.requiresUnpack);
 		try {