update BUILD files

vendor/github.com/coreos/etcd/mvcc/BUILD

@@ -16,6 +16,7 @@ go_library(
         "watcher.go",
         "watcher_group.go",
     ],
+    importpath = "github.com/coreos/etcd/mvcc",
     visibility = ["//visibility:public"],
     deps = [
         "//vendor/github.com/coreos/etcd/lease:go_default_library",

vendor/github.com/coreos/etcd/mvcc/backend/BUILD

@@ -14,6 +14,7 @@ go_library(
         ],
         "//conditions:default": [],
     }),
+    importpath = "github.com/coreos/etcd/mvcc/backend",
     visibility = ["//visibility:public"],
     deps = [
         "//vendor/github.com/boltdb/bolt:go_default_library",

vendor/github.com/coreos/etcd/mvcc/mvccpb/BUILD

@@ -3,6 +3,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library")
 go_library(
     name = "go_default_library",
     srcs = ["kv.pb.go"],
+    importpath = "github.com/coreos/etcd/mvcc/mvccpb",
     visibility = ["//visibility:public"],
     deps = ["//vendor/github.com/golang/protobuf/proto:go_default_library"],
 )