Correct file type of module descriptors included in JAR files

Prior to this commit, all JAR files generated for a published module
included their compiled module descriptor within the root directory
of the artifact.

This commit a) includes the compiled module descriptor only in JAR
files that have no classifier specified and b) stores the source of
each module descriptor within the matching `*-source.jar` file.

Fixes #2003

buildSrc/src/main/kotlin/java-library-conventions.gradle.kts

@@ -80,6 +80,9 @@ if (project in mavenizedProjects) {
 		dependsOn(tasks.classes)
 		archiveClassifier.set("sources")
 		from(sourceSets.main.get().allSource)
+		from("${project.projectDir}/src/module/$javaModuleName") {
+			include("module-info.java")
+		}
 		duplicatesStrategy = DuplicatesStrategy.EXCLUDE
 	}
 
@@ -93,8 +96,11 @@ if (project in mavenizedProjects) {
 			include("LICENSE.md", "LICENSE-notice.md")
 			into("META-INF")
 		}
-		from("$buildDir/classes/java/module/$javaModuleName") {
-			include("module-info.class")
+		val suffix = archiveClassifier.getOrElse("")
+		if (suffix.isBlank() || suffix == "all") { // "all" is used by shadow plugin
+			from("$buildDir/classes/java/module/$javaModuleName") {
+				include("module-info.class")
+			}
 		}
 	}