Add a package listing API to cipd.

go/src/infra/tools/cipd/client.go

Index: go/src/infra/tools/cipd/client.go
diff --git a/go/src/infra/tools/cipd/client.go b/go/src/infra/tools/cipd/client.go
index e6e0c8906082fe1047ef43e2245f4b5598e4e87d..b67ee869e1bb740ace15dcabceeb5f8369efaf65 100644
--- a/go/src/infra/tools/cipd/client.go
+++ b/go/src/infra/tools/cipd/client.go
@@ -167,6 +167,9 @@ type Client interface {
 	// the site root. It doesn't check whether the instance is already deployed.
 	FetchAndDeployInstance(pin common.Pin) error
 
+	// ListPackages returns a list of strings of package names.
+	ListPackages(prefix string) ([]string, error)
+
 	// ProcessEnsureFile parses text file that describes what should be installed
 	// by EnsurePackages function. It is a text file where each line has a form:
 	// <package name> <desired version>. Whitespaces are ignored. Lines that start
@@ -278,6 +281,10 @@ func (client *clientImpl) ModifyACL(packagePath string, changes []PackageACLChan
 	return client.remote.modifyACL(packagePath, changes)
 }
 
+func (client *clientImpl) ListPackages(prefix string) ([]string, error) {
+	return client.remote.listPackages(prefix)
+}
+
 func (client *clientImpl) UploadToCAS(sha1 string, data io.ReadSeeker, session *UploadSession) error {
 	// Open new upload session if an existing is not provided.
 	var err error
@@ -596,6 +603,8 @@ type remote interface {
 
 	attachTags(pin common.Pin, tags []string) error
 	fetchInstance(pin common.Pin) (*fetchInstanceResponse, error)
+
+	listPackages(prefix string) ([]string, error)
 }
 
 type storage interface {