Move scala/{private/macros/,}toolchains_repo.bzl

Like "Move `scala_toolchains` to `scala/toolchains.bzl`",
removes the `scala_toolchains_repo` symbol from `scala/scala.bzl` and
makes it available from `scala/toolchains_repo.bzl`.

This avoids a future `test_scala_version 2.12.20` failure during Bazel 8
builds after adding `twitter_scrooge` toolchain support in the new
`test_version/version_specific_tests_dir/scrooge_repositories.bzl` file.
Otherwise, this new file would load `toolchains_repo` from
`scala/scala.bzl`. The `test_version/test_scala_version_.../WORKSPACE`
file generated from `test_version/WORKSPACE.template` would then
transitively load `.bzl` files with `rules_java` symbols, breaking the
test.

```txt
$ RULES_SCALA_TEST_ONLY="test_scala_version 2.12.20" ./test_version.sh

ERROR: Traceback (most recent call last):
  File ".../external/rules_scala/scala/private/common_attributes.bzl",
  line 18, column 28, in <toplevel>
    "deps": attr.label_list(

Error in label_list:
  Illegal argument: element in 'providers' is of unexpected type.
  Either all elements should be providers,
  or all elements should be lists of providers,
  but got list with an element of type NoneType.

ERROR: Error computing the main repository mapping:
  at test_version/test_scala_version_.../scrooge_repositories.bzl:1:6:
  at .../scala/scala.bzl:30:5:
  at .../scala/private/rules/scala_junit_test.bzl:5:5:
initialization of module 'scala/private/common_attributes.bzl' failed
```

scala/scala.bzl

@@ -2,10 +2,6 @@ load(
     "//specs2:specs2_junit.bzl",
     _specs2_junit_dependencies = "specs2_junit_dependencies",
 )
-load(
-    "//scala/private:macros/toolchains_repo.bzl",
-    _scala_toolchains_repo = "scala_toolchains_repo",
-)
 load(
     "//scala/private:macros/scala_repositories.bzl",
     _rules_scala_setup = "rules_scala_setup",
@@ -80,4 +76,3 @@ rules_scala_toolchain_deps_repositories = _rules_scala_toolchain_deps_repositori
 scala_test = _scala_test
 scala_test_suite = _scala_test_suite
 setup_scala_toolchain = _setup_scala_toolchain
-scala_toolchains_repo = _scala_toolchains_repo

scala/toolchains.bzl

@@ -2,7 +2,7 @@
 
 load("//junit:junit.bzl", "junit_artifact_ids")
 load("//scala/private:macros/scala_repositories.bzl", "scala_repositories")
-load("//scala/private:macros/toolchains_repo.bzl", "scala_toolchains_repo")
+load("//scala:toolchains_repo.bzl", "scala_toolchains_repo")
 load("//scala:scala_cross_version.bzl", "default_maven_server_urls")
 load("//scalatest:scalatest.bzl", "scalatest_artifact_ids")
 load("//specs2:specs2.bzl", "specs2_artifact_ids")