Give the .jar extension to maven-plugin packages

core/shared/src/main/scala/coursier/maven/MavenSource.scala

@@ -152,6 +152,7 @@ object MavenSource {
   
   val typeExtensions: Map[String, String] = Map(
     "eclipse-plugin" -> "jar",
+    "maven-plugin"   -> "jar",
     "hk2-jar"        -> "jar",
     "orbit"          -> "jar",
     "scala-jar"      -> "jar",

tests/shared/src/test/scala/coursier/test/CentralTests.scala

@@ -247,6 +247,15 @@ object CentralTests extends TestSuite {
           "jar"
         )
       }
+
+      'mavenPlugin - {
+        // has packaging maven-plugin - ensuring coursier gives its artifact the .jar extension
+        ensureArtifactHasExtension(
+          Module("org.bytedeco", "javacpp"),
+          "1.1",
+          "jar"
+        )
+      }
     }
   }