bazel: add pkg/cmd/mirror

This tool will be extended with functionality to mirror Go module ZIP's
to cloud storage. For now, the mirroring functionality is missing, so
we just generate the `DEPS.bzl` content in much the same way as
`gazelle update-repos` does. (This doesn't change the content of
`DEPS.bzl` in any way besides alphabetizing the contents.)

Release note: None

.github/CODEOWNERS

@@ -166,6 +166,7 @@
 /pkg/cmd/gossipsim/          @cockroachdb/kv-prs
 /pkg/cmd/import-tools/       @cockroachdb/dev-inf
 /pkg/cmd/internal/issues/    @cockroachdb/test-eng
+/pkg/cmd/mirror/             @cockroachdb/dev-inf
 /pkg/cmd/prereqs/            @cockroachdb/dev-inf
 /pkg/cmd/protoc-gen-gogoroach/ @cockroachdb/server-prs
 /pkg/cmd/publish-artifacts/  @cockroachdb/dev-inf

BUILD.bazel

@@ -1,3 +1,8 @@
+exports_files([
+    "go.mod",
+    "go.sum",
+])
+
 load("@bazel_gazelle//:def.bzl", "gazelle")
 
 # The following directives inform gazelle how to auto-generate BUILD.bazel