Fix SWIFT_TOOLCHAIN_TYPE to work with Bzlmod + AEG

Use a label to it resolves correctly regardless of the repo that references it.

Signed-off-by: Brentley Jones <github@brentleyjones.com>

Author: brentleyjones

swift/internal/toolchain_utils.bzl

@@ -14,7 +14,7 @@
 
 """Helpers used to depend on and access the Swift toolchain."""
 
-SWIFT_TOOLCHAIN_TYPE = "@build_bazel_rules_swift//toolchains:toolchain_type"
+SWIFT_TOOLCHAIN_TYPE = Label("//toolchains:toolchain_type")
 
 def get_swift_toolchain(
         ctx,