Give scala_toolchains_repo a default name argument

This turns out to be helpful when adapting
`test_version/WORKSPACE.template` to the toolchainized version of
`twitter_scrooge` for testing. In this case, we want `twitter_scooge` to
be in its own customized repo, separate from the one generated by
`scala_toolchains`.

This seems like it might be generally useful when writing module
extensions for alternative toolchains.

Author: mbland

scala/private/macros/toolchains_repo.bzl

@@ -27,9 +27,9 @@ _scala_toolchains_repo = repository_rule(
     },
 )
 
-def scala_toolchains_repo(**kwargs):
+def scala_toolchains_repo(name = "io_bazel_rules_scala_toolchains", **kwargs):
     _scala_toolchains_repo(
-        name = "io_bazel_rules_scala_toolchains",
+        name = name,
         **kwargs
     )