WIP: Bzlmod part the forty-ninth

Made the config extension aware of the `SCALA_VERSION` and
`ENABLE_COMPILER_DEPENDENCY_TRACKING` env vars to fix part of
`test_classpath_contains_2_12` from test/shell/test_scala_config.sh.

Now it's dying with:

```txt
Analyzing: target //src/java/io/bazel/rulesscala/scalac:scalac
  (1 packages loaded, 0 targets configured)

ERROR: Traceback (most recent call last):
File ".../scala/extensions/deps.bzl", line 210, column 30,
  in _scala_deps_impl
    scalafmt_repositories(
File ".../scala/scalafmt/scalafmt_repositories.bzl", line 67, column 21,
  in scalafmt_repositories
    repositories(
File ".../third_party/repositories/repositories.bzl", line 103, column 33,
  in repositories
    artifact = artifacts[id]["artifact"],

Error: key "com_geirsson_metaconfig_pprint" not found in dictionary
ERROR: Analysis of target '//src/java/io/bazel/rulesscala/scalac:scalac'
  failed; build aborted: error evaluating module extension scala_deps in
  //scala/extensions:deps.bzl
```

Still, getting closer and closer to getting all the tests to pass under
Bzlmod!

scala/extensions/config.bzl

@@ -12,15 +12,24 @@ _settings = tag_class(
 )
 
 def _get_root_settings(module_ctx):
+    scala_version = DEFAULT_SCALA_VERSION
+    compiler_dep_tracking = False
     root_settings = module_ctx.modules[0].tags.settings
 
-    if len(root_settings) == 0:
-        return DEFAULT_SCALA_VERSION, False
-    root = root_settings[0]
-    return root.scala_version, root.enable_compiler_dependency_tracking
+    if len(root_settings) != 0:
+        root = root_settings[0]
+        scala_version = root.scala_version
+        compiler_dep_tracking = root.enable_compiler_dependency_tracking
 
-def _collect_versions(module_ctx):
-    versions = {}
+    return (
+        module_ctx.os.environ.get("SCALA_VERSION", scala_version),
+        module_ctx.os.environ.get(
+            "ENABLE_COMPILER_DEPENDENCY_TRACKING", compiler_dep_tracking
+        ),
+    )
+
+def _collect_versions(module_ctx, scala_version):
+    versions = {scala_version: None}
 
     for mod in module_ctx.modules:
         for settings in mod.tags.settings:
@@ -33,7 +42,7 @@ def _scala_config_impl(module_ctx):
 
     _scala_config(
         scala_version = version,
-        scala_versions = _collect_versions(module_ctx),
+        scala_versions = _collect_versions(module_ctx, version),
         enable_compiler_dependency_tracking = compiler_dep_tracking,
     )
     return module_ctx.extension_metadata(
@@ -44,4 +53,5 @@ def _scala_config_impl(module_ctx):
 scala_config = module_extension(
     implementation = _scala_config_impl,
     tag_classes = {"settings": _settings},
+    environ = ["SCALA_VERSION", "ENABLE_COMPILER_DEPENDENCY_TRACKING"],
 )