Merge pull request #1517 from apache/tristan/source-api-kwargs

source.py: Remove ambiguous kwargs from public API signature
diff --git a/src/buildstream/source.py b/src/buildstream/source.py
index 01bc270..bd4b3ec 100644
--- a/src/buildstream/source.py
+++ b/src/buildstream/source.py
@@ -424,7 +424,7 @@
         """
         raise ImplError("Source plugin '{}' does not implement set_ref()".format(self.get_kind()))
 
-    def track(self, **kwargs) -> SourceRef:
+    def track(self, *, previous_sources_dir: str = None) -> SourceRef:
         """Resolve a new ref from the plugin's track option
 
         Args:
@@ -452,7 +452,7 @@
         # Allow a non implementation
         return None
 
-    def fetch(self, **kwargs) -> None:
+    def fetch(self, *, previous_sources_dir: str = None) -> None:
         """Fetch remote sources and mirror them locally, ensuring at least
         that the specific reference is cached locally.