WIP: Bzlmod part the one hundred-sixteenth

Move testing repos to dev_deps, update tag reading

As I was about to work on `WORKSPACE` compatibiity, I realized we
shouldn't bake `rules_scala` testing repos into the main `deps.bzl`
module extension. In the process of moving them to the internal
`dev_deps.bzl` extension, I rethought my earlier tag reading design. The
new version seems more correct and readable.

Author: mbland

MODULE.bazel

@@ -88,29 +88,22 @@ dev_deps.toolchains(
     twitter_scrooge = True,
 )
 
-# Default versions of version specific repos needed by some of our tests. Tests
-# that set `--repo_env=SCALA_VERSION=...` break without using the default here,
-# because version specific repos for other versions won't be available.
 use_repo(
     dev_deps,
-    "io_bazel_rules_scala_guava",
+    "scala_proto_rules_protoc_bridge",
+    "scala_proto_rules_scalapb_plugin",
+    "scalafmt_default",
 )
 
-# See //testing/private:repositories.bzl for notes on some of these repos.
+# Default versions of version specific repos needed by some of our tests. Tests
+# that set `--repo_env=SCALA_VERSION=...` break without using the default here,
+# because version specific repos for other versions won't be available.
 use_repo(
     dev_deps,
-    "com_github_jnr_jffi_native",
-    "com_google_guava_guava_21_0",
-    "com_google_guava_guava_21_0_with_file",
-    "com_twitter__scalding_date",
-    "org_apache_commons_commons_lang_3_5",
-    "org_springframework_spring_core",
-    "org_springframework_spring_tx",
-    "org_typelevel__cats_core",
-    "org_typelevel_kind_projector",
-    "scala_proto_rules_protoc_bridge",
-    "scala_proto_rules_scalapb_plugin",
-    "scalafmt_default",
+    "io_bazel_rules_scala_guava",
+    "io_bazel_rules_scala_junit_junit",
+    "io_bazel_rules_scala_scala_compiler",
+    "io_bazel_rules_scala_scala_library",
 )
 
 [
@@ -140,6 +133,21 @@ internal_dev_deps = use_extension(
     "dev_deps",
     dev_dependency = True,
 )
+
+# See //testing/private:repositories.bzl for notes on some of these repos.
+use_repo(
+    internal_dev_deps,
+    "com_github_jnr_jffi_native",
+    "com_google_guava_guava_21_0",
+    "com_google_guava_guava_21_0_with_file",
+    "com_twitter__scalding_date",
+    "org_apache_commons_commons_lang_3_5",
+    "org_springframework_spring_core",
+    "org_springframework_spring_tx",
+    "org_typelevel__cats_core",
+    "org_typelevel_kind_projector",
+)
+
 use_repo(
     internal_dev_deps,
     "com_github_bazelbuild_buildtools",