Expose extension for Git toolchain (#967)

This move all toolchains into the rules_pkg_git repo.

* Expose extension
* Use extension in MODULE.bazel

Author: guw

MODULE.bazel

@@ -25,6 +25,15 @@ register_toolchains(
     dev_dependency = True,
 )
 
+# Find the system Git if one is available.
+find_git = use_extension("//toolchains/git:git_configure.bzl", "experimental_find_system_git_extension", dev_dependency = True)
+use_repo(find_git, "rules_pkg_git")
+
+register_toolchains(
+    "@rules_pkg_git//:all",
+    dev_dependency = True,
+)
+
 local_repository = use_repo_rule("@bazel_tools//tools/build_defs/repo:local.bzl", "local_repository")
 
 local_repository(

WORKSPACE.bzlmod

@@ -13,16 +13,3 @@ local_repository(
 #    name = "rules_pkg_rpmbuild",
 #    verbose = False,
 #)
-
-# Needed for making our release notes
-load("//toolchains/git:git_configure.bzl", "experimental_find_system_git_bzlmod")
-
-experimental_find_system_git_bzlmod(
-    name = "rules_pkg_git",
-    verbose = False,
-)
-
-register_toolchains(
-    "@rules_pkg_git//:git_auto_toolchain",
-    "//toolchains/git:git_missing_toolchain",
-)

toolchains/git/BUILD

@@ -67,9 +67,3 @@ toolchain_type(
 git_toolchain(
     name = "no_git",
 )
-
-toolchain(
-    name = "git_missing_toolchain",
-    toolchain = ":no_git",
-    toolchain_type = ":git_toolchain_type",
-)

toolchains/git/BUILD.tpl

@@ -14,3 +14,9 @@ toolchain(
     toolchain = ":git_auto",
     toolchain_type = "@rules_pkg//toolchains/git:git_toolchain_type",
 )
+
+toolchain(
+    name = "zzz_git_missing_toolchain",  # keep name lexicographically last
+    toolchain = "@rules_pkg//toolchains/git:no_git",
+    toolchain_type = "@rules_pkg//toolchains/git:git_toolchain_type",
+)

toolchains/git/git_configure.bzl

@@ -74,8 +74,7 @@ def experimental_find_system_git(name, workspace_file = None, verbose = False):
         workspace_file = Label("//:WORKSPACE")
     _find_system_git(name = name, workspace_file = workspace_file, verbose = verbose)
     native.register_toolchains(
-        "@%s//:git_auto_toolchain" % name,
-        "@rules_pkg//toolchains/git:git_missing_toolchain",
+        "@%s//:all" % name,
     )
 
 # buildifier: disable=function-docstring-args
@@ -93,3 +92,8 @@ def experimental_find_system_git_bzlmod(name, workspace_file = None, verbose = F
     if not workspace_file:
         workspace_file = Label("//:MODULE.bazel")
     _find_system_git(name = name, workspace_file = workspace_file, verbose = verbose)
+
+# For use from MODULE.bzl
+experimental_find_system_git_extension = module_extension(
+    implementation = lambda ctx: _find_system_git(name = "rules_pkg_git", workspace_file = Label("//:MODULE.bazel")),
+)