Update phase_scalafmt.md, check scalafmt conf path

Updates the Scalafmt documentation to reflect the current API. Adds a
check to `scala_toolchains_repo` to `fail` if the Scalafmt
`default_config` file doesn't exist.

The previous commit doesn't actually restore the exact pre-bazel-contrib#1725 API.
It eliminates the `exports_files` requirement, but still requires a
`Label`, not just a relative path string or a `.scalafmt.conf` in the
root directory.

After experimenting a bit and thinking this through, it seems the
explicit `Label` requirement provides the most robust and reliable
interface. Specifically, supporting the previously optional
`.scalafmt.conf` in the root directory requires detecting whether it
actually exists before falling back to the default. Having users
explicitly specify their own config seems a small burden to impose for a
more straightforward and correct implementation.

At the same time, I saw the opportunity to provide the user with
explicit feedback if the specified config file doesn't exist. Hence the
new check and `fail()` message.

Also renamed the generated `.scalafmt.conf` file in
`@rules_scala_toolchains//scalafmt` to `scalafmt.conf`. No need for it
to be a hidden file in that context.

Author: mbland

docs/phase_scalafmt.md

@@ -71,24 +71,21 @@ bazel run <TARGET>.format-test
 
 to check the format (without modifying source code).
 
-The extension provides a default configuration, but there are 2 ways to use
-a custom configuration:
-
-- Put `.scalafmt.conf` at the root of your workspace
-- Pass `.scalafmt.conf` into the toolchain configuration:
-
-    ```py
-    # MODULE.bazel
-    scala_deps.scalafmt(
-        default_config = "//path/to/my/custom:scalafmt.conf",
-    )
-
-    # WORKSPACE
-    scala_toolchains(
-        # Other toolchains settings...
-        scalafmt = {"default_config": "//path/to/my/custom:scalafmt.conf"},
-    )
-    ```
+The extension provides a default configuration. To use a custom configuration,
+pass its path or target Label to the toolchain configuration:
+
+```py
+# MODULE.bazel
+scala_deps.scalafmt(
+    default_config = "path/to/my/custom/scalafmt.conf",
+)
+
+# WORKSPACE
+scala_toolchains(
+    # Other toolchains settings...
+    scalafmt = {"default_config": "path/to/my/custom/scalafmt.conf"},
+)
+```
 
 When using Scala 3, you must append `runner.dialect = scala3` to
 `.scalafmt.conf`.

scala/toolchains_repo.bzl

@@ -68,10 +68,11 @@ def _scala_toolchains_repo_impl(repository_ctx):
 
     if repo_attr.scalafmt:
         toolchains["scalafmt"] = _SCALAFMT_TOOLCHAIN_BUILD
-        repository_ctx.symlink(
-            repository_ctx.path(repo_attr.scalafmt_default_config),
-            "scalafmt/.scalafmt.conf",
-        )
+        config_path = repository_ctx.path(repo_attr.scalafmt_default_config)
+
+        if not config_path.exists:
+            fail("Scalafmt default config file doesn't exist:", config_path)
+        repository_ctx.symlink(config_path, "scalafmt/scalafmt.conf")
 
     # Generate a root package so that the `register_toolchains` call in
     # `MODULE.bazel` always succeeds.
@@ -190,7 +191,7 @@ load(
 
 filegroup(
     name = "config",
-    srcs = [":.scalafmt.conf"],
+    srcs = [":scalafmt.conf"],
     visibility = ["//visibility:public"],
 )