Update library package to point to google.golang.org/protobuf/proto

• appengine.go[diff]
• blobstore/blobstore.go[diff]
• blobstore/read.go[diff]
• datastore/datastore.go[diff]
• datastore/internal/cloudkey/cloudkey.go[diff]
• datastore/internal/cloudpb/entity.pb.go[diff]
• datastore/key.go[diff]
• datastore/load.go[diff]
• datastore/load_test.go[diff]
• datastore/query.go[diff]
• datastore/query_test.go[diff]
• datastore/save.go[diff]
• delay/delay_test.go[diff]
• go.mod[diff]
• go.sum[diff]
• internal/aetesting/fake.go[diff]
• internal/api.go[diff]
• internal/api_classic.go[diff]
• internal/api_common.go[diff]
• internal/api_test.go[diff]
• internal/app_identity/app_identity_service.pb.go[diff]
• internal/base/api_base.pb.go[diff]
• internal/blobstore/blobstore_service.pb.go[diff]
• internal/capability/capability_service.pb.go[diff]
• internal/channel/channel_service.pb.go[diff]
• internal/datastore/datastore_v3.pb.go[diff]
• internal/image/images_service.pb.go[diff]
• internal/internal.go[diff]
• internal/log/log_service.pb.go[diff]
• internal/mail/mail_service.pb.go[diff]
• internal/modules/modules_service.pb.go[diff]
• internal/remote_api/remote_api.pb.go[diff]
• internal/search/search.pb.go[diff]
• internal/socket/socket_service.pb.go[diff]
• internal/system/system_service.pb.go[diff]
• internal/taskqueue/taskqueue_service.pb.go[diff]
• internal/transaction.go[diff]
• internal/urlfetch/urlfetch_service.pb.go[diff]
• internal/user/user_service.pb.go[diff]
• internal/xmpp/xmpp_service.pb.go[diff]
• log/log.go[diff]
• log/log_test.go[diff]
• mail/mail.go[diff]
• mail/mail_test.go[diff]
• memcache/memcache.go[diff]
• module/module.go[diff]
• module/module_test.go[diff]
• remote_api/client.go[diff]
• remote_api/remote_api.go[diff]
• runtime/runtime_test.go[diff]
• search/search.go[diff]
• search/search_test.go[diff]
• socket/socket_classic.go[diff]
• taskqueue/taskqueue.go[diff]
• urlfetch/urlfetch.go[diff]
• user/user.go[diff]
• user/user_test.go[diff]
• v2/appengine.go[diff]
• v2/blobstore/blobstore.go[diff]
• v2/blobstore/read.go[diff]
• v2/datastore/datastore.go[diff]
• v2/datastore/internal/cloudkey/cloudkey.go[diff]
• v2/datastore/internal/cloudpb/entity.pb.go[diff]
• v2/datastore/key.go[diff]
• v2/datastore/load.go[diff]
• v2/datastore/load_test.go[diff]
• v2/datastore/query.go[diff]
• v2/datastore/query_test.go[diff]
• v2/datastore/save.go[diff]
• v2/delay/delay_test.go[diff]
• v2/go.mod[diff]
• v2/go.sum[diff]
• v2/internal/aetesting/fake.go[diff]
• v2/internal/api.go[diff]
• v2/internal/api_common.go[diff]
• v2/internal/api_test.go[diff]
• v2/internal/internal.go[diff]
• v2/internal/transaction.go[diff]
• v2/mail/mail.go[diff]
• v2/mail/mail_test.go[diff]
• v2/memcache/memcache.go[diff]
• v2/module/module.go[diff]
• v2/module/module_test.go[diff]
• v2/runtime/runtime_test.go[diff]
• v2/search/search.go[diff]
• v2/search/search_test.go[diff]
• v2/taskqueue/taskqueue.go[diff]
• v2/urlfetch/urlfetch.go[diff]
• v2/user/user.go[diff]
• v2/user/user_test.go[diff]
• xmpp/xmpp_test.go[diff]