Replace Scalafmt default_config alias with symlink

This avoids the need for the user to use `exports_files` so
`@rules_scala_toolchains//scalafmt:config` can access the config file.
Essentially restores the API from before bazel-contrib#1725, but still fixes the same
bug as bazel-contrib#1725.

Author: mbland

BUILD

@@ -1 +0,0 @@
-exports_files([".scalafmt.conf"])

scala/extensions/deps.bzl

@@ -96,6 +96,7 @@ _scalafmt_attrs = {
     "default_config": attr.label(
         default = _scalafmt_defaults["default_config"],
         doc = "The default config file for Scalafmt targets",
+        allow_single_file = True,
     ),
 }
 

scala/toolchains_repo.bzl

@@ -68,8 +68,9 @@ def _scala_toolchains_repo_impl(repository_ctx):
 
     if repo_attr.scalafmt:
         toolchains["scalafmt"] = _SCALAFMT_TOOLCHAIN_BUILD
-        format_args["scalafmt_default_config"] = (
-            repo_attr.scalafmt_default_config
+        repository_ctx.symlink(
+            repository_ctx.path(repo_attr.scalafmt_default_config),
+            "scalafmt/.scalafmt.conf",
         )
 
     # Generate a root package so that the `register_toolchains` call in
@@ -187,9 +188,9 @@ load(
     "setup_scalafmt_toolchains",
 )
 
-alias(
+filegroup(
     name = "config",
-    actual = "{scalafmt_default_config}",
+    srcs = [":.scalafmt.conf"],
     visibility = ["//visibility:public"],
 )