Update _jvm_import_external JAR alias target

Changes the target for the `//jar:jar` alias in `_jvm_import_scala` to
the top level repo target directly (`//:%s`) instead of the repo name
(`@%s`).

Functionally the same, but seems a bit cleaner than referencing the
target as though it were external to the repo.

scala/scala_maven_import_external.bzl

@@ -138,7 +138,7 @@ def _jvm_import_external(repository_ctx):
         "",
         "alias(",
         "    name = \"jar\",",
-        "    actual = \"@%s\"," % repo_name,
+        "    actual = \"//:%s\"," % repo_name,
         ")",
         "",
     ]))