Enable users to customize cmake and ninja versions with bzlmod

Avoid hardcoding cmake and ninja versions in repository names that the
bzlmod extension produces.

Author: mrkkrp

MODULE.bazel

@@ -18,15 +18,15 @@ bazel_dep(name = "rules_cc", version = "0.0.9", dev_dependency = True)
 tools = use_extension("@rules_foreign_cc//foreign_cc:extensions.bzl", "tools")
 use_repo(
     tools,
-    "cmake_3.23.2_toolchains",
     "cmake_src",
+    "cmake_toolchains",
     "gettext_runtime",
     "glib_dev",
     "glib_src",
     "gnumake_src",
     "meson_src",
-    "ninja_1.11.1_toolchains",
     "ninja_build_src",
+    "ninja_toolchains",
     "pkgconfig_src",
     "rules_foreign_cc_framework_toolchains",
 )
@@ -40,6 +40,6 @@ register_toolchains(
     "@rules_foreign_cc//toolchains:preinstalled_automake_toolchain",
     "@rules_foreign_cc//toolchains:preinstalled_m4_toolchain",
     "@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain",
-    "@cmake_3.23.2_toolchains//:all",
-    "@ninja_1.11.1_toolchains//:all",
+    "@cmake_toolchains//:all",
+    "@ninja_toolchains//:all",
 )

foreign_cc/extensions.bzl

@@ -23,22 +23,31 @@ def _init(module_ctx):
         register_built_pkgconfig_toolchain = True,
     )
 
-    versions = {
-        "cmake": _DEFAULT_CMAKE_VERSION,
-        "ninja": _DEFAULT_NINJA_VERSION,
-    }
+    cmake_version = _DEFAULT_CMAKE_VERSION
+    ninja_version = _DEFAULT_NINJA_VERSION
 
+    # Traverse all modules starting from the root one (the first in
+    # module_ctx.modules). The first occurrence of cmake or ninja tag wins.
+    # Multiple versions requested from the same module are rejected.
     for mod in module_ctx.modules:
-        if not mod.is_root:
-            for toolchain in mod.tags.cmake:
-                versions["cmake"] = toolchain.version
+        cmake_versions_count = len(mod.tags.cmake)
+        if cmake_versions_count == 1:
+            cmake_version = mod.tags.cmake[0].version
+            break
+        elif cmake_versions_count > 1:
+            fail("More than one cmake version requested: {}".format(mod.tags.cmake))
 
-            for toolchain in mod.tags.ninja:
-                versions["ninja"] = toolchain.version
+    for mod in module_ctx.modules:
+        ninja_versions_count = len(mod.tags.ninja)
+        if ninja_versions_count == 1:
+            ninja_version = mod.tags.ninja[0].version
+            break
+        elif ninja_versions_count > 1:
+            fail("More than one ninja version requested: {}".format(mod.tags.ninja))
 
     prebuilt_toolchains(
-        cmake_version = versions["cmake"],
-        ninja_version = versions["ninja"],
+        cmake_version = cmake_version,
+        ninja_version = ninja_version,
         register_toolchains = False,
     )