LocateUri: pass URI plugin kind, optionally disables plugin verify

Commit b3a458338a added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

src/LocateUri.hxx

@@ -41,6 +41,12 @@ class Client;
 class Storage;
 #endif
 
+enum class UriPluginKind {
+	INPUT,
+	STORAGE,
+	PLAYLIST,
+};
+
 struct LocatedUri {
 	enum class Type {
 		/**
@@ -84,7 +90,8 @@ struct LocatedUri {
  * that feature is disabled if this parameter is nullptr
  */
 LocatedUri
-LocateUri(const char *uri, const Client *client
+LocateUri(UriPluginKind kind,
+	  const char *uri, const Client *client
 #ifdef ENABLE_DATABASE
 	  , const Storage *storage
 #endif