Add a package listing API to cipd.

go/src/infra/tools/cipd/remote.go

Index: go/src/infra/tools/cipd/remote.go
diff --git a/go/src/infra/tools/cipd/remote.go b/go/src/infra/tools/cipd/remote.go
index 8856d7ce57a9a8d366183939b511dd036e26712d..e395c5c0563afae2970cde76c77f0d54f037cc3a 100644
--- a/go/src/infra/tools/cipd/remote.go
+++ b/go/src/infra/tools/cipd/remote.go
@@ -418,6 +418,30 @@ func (r *remoteImpl) attachTags(pin common.Pin, tags []string) error {
 	return fmt.Errorf("Unexpected status when attaching tags: %s", reply.Status)
 }
 
+func (r *remoteImpl) listPackages(prefix string) ([]string, error) {
+	endpoint, err := packageEndpoint(prefix)
+	if err != nil {
+		return nil, err
+	}
+	var reply struct {
+		Status       string   `json:"status"`
+		ErrorMessage string   `json:"error_message"`
+		Packages     []string `json:"packages"`
+	}
+	err = r.makeRequest(endpoint, "GET", nil, &reply)
+	if err != nil {
+		return nil, err
+	}
+	switch reply.Status {
+	case "SUCCESS":
+		out := reply.Packages
+		return out, nil
+	case "ERROR":
+		return nil, errors.New(reply.ErrorMessage)
+	}
+	return nil, fmt.Errorf("Unexpected list packages status: %s", reply.Status)
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 func instanceEndpoint(pin common.Pin) (string, error) {
@@ -441,6 +465,12 @@ func aclEndpoint(packagePath string) (string, error) {
 	return "repo/v1/acl?" + params.Encode(), nil
 }
 
+func packageEndpoint(prefix string) (string, error) {
+	params := url.Values{}
+	params.Add("prefix", prefix)
+	return "repo/v1/packages?" + params.Encode(), nil
+}
+
 func tagsEndpoint(pin common.Pin, tags []string) (string, error) {
 	err := common.ValidatePin(pin)
 	if err != nil {