swift_rules_extra_dependencies() fails with rules_proto 6.0.0

[Jiawen (Kevin) Chen]

The recent rules_proto 6.0.0 release migrates rules_proto_toolchains() from repositories.bzl to toolchains.bzl. We need a migration path for swift/extras.bzl.

Current:

load(
    "@rules_proto//proto:repositories.bzl",
    "rules_proto_dependencies",
    "rules_proto_toolchains",
)

Should be:

load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies")
load("@rules_proto//proto:toolchains.bzl", "rules_proto_toolchains")