migration: rework migration subsystem into multiple packages

No tests were harmed in this commit. Some abstractions which felt like cruft
were thinned.

Release note: None

pkg/migration/BUILD.bazel

@@ -1,70 +1,20 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "migration",
     srcs = [
-        "helper.go",
-        "manager.go",
-        "migrations.go",
-        "util.go",
+        "helpers.go",
+        "migration.go",
+        "registry.go",
     ],
     importpath = "github.com/cockroachdb/cockroach/pkg/migration",
     visibility = ["//visibility:public"],
     deps = [
         "//pkg/clusterversion",
-        "//pkg/keys",
         "//pkg/kv",
-        "//pkg/kv/kvserver/liveness/livenesspb",
         "//pkg/roachpb",
-        "//pkg/rpc",
-        "//pkg/rpc/nodedialer",
         "//pkg/server/serverpb",
-        "//pkg/sql",
-        "//pkg/sql/sqlutil",
-        "//pkg/util/ctxgroup",
         "//pkg/util/log",
-        "//pkg/util/quotapool",
-        "@com_github_cockroachdb_errors//:errors",
         "@com_github_cockroachdb_logtags//:logtags",
-        "@com_github_cockroachdb_redact//:redact",
-        "@org_golang_google_grpc//:go_default_library",
-    ],
-)
-
-go_test(
-    name = "migration_test",
-    srcs = [
-        "client_test.go",
-        "helper_test.go",
-        "main_test.go",
-        "migrations_test.go",
-        "util_test.go",
-    ],
-    embed = [":migration"],
-    deps = [
-        "//pkg/base",
-        "//pkg/clusterversion",
-        "//pkg/kv",
-        "//pkg/kv/kvserver",
-        "//pkg/kv/kvserver/batcheval",
-        "//pkg/kv/kvserver/liveness",
-        "//pkg/kv/kvserver/liveness/livenesspb",
-        "//pkg/kv/kvserver/stateloader",
-        "//pkg/roachpb",
-        "//pkg/security",
-        "//pkg/security/securitytest",
-        "//pkg/server",
-        "//pkg/server/serverpb",
-        "//pkg/settings/cluster",
-        "//pkg/sql/tests",
-        "//pkg/testutils",
-        "//pkg/testutils/serverutils",
-        "//pkg/testutils/testcluster",
-        "//pkg/util/leaktest",
-        "//pkg/util/log",
-        "//pkg/util/syncutil",
-        "@com_github_cockroachdb_errors//:errors",
-        "@com_github_stretchr_testify//require",
-        "@org_golang_google_grpc//:go_default_library",
     ],
 )