Allow source provenance info to be overridden

This allows multi-source source plugins to provide
this information per source rather than as a
singular top level.

This is done by adding source_provenance_attrs
arg to create_source_info() that when specified
overrides the use of the source's top level source
provenance info

Author: joshua-zivkovic

src/buildstream/source.py

@@ -1367,6 +1367,7 @@ def create_source_info(
         version: str,
         *,
         version_guess: Optional[str] = None,
+        source_provenance_attrs: Optional[SourceProvenance] = None,
         extra_data: Optional[Dict[str, str]] = None,
     ) -> SourceInfo:
         """Create a :class:`.SourceInfo` object
@@ -1392,9 +1393,18 @@ def create_source_info(
         """
         homepage = None
         issue_tracker = None
-        if self.__provenance is not None:
-            homepage = self.__provenance.homepage
-            issue_tracker = self.__provenance.issue_tracker
+
+        if (
+            self.__provenance is not None
+            or source_provenance_attrs is not None
+        ):
+            if source_provenance_attrs is None:
+                provenance_source = self.__provenance
+            else:
+                provenance_source = source_provenance_attrs
+
+            homepage = provenance_source.homepage
+            issue_tracker = provenance_source.issue_tracker
 
         return SourceInfo(
             self.get_kind(),

src/buildstream/types.py

@@ -211,6 +211,17 @@ def new_from_node(cls, node: MappingNode) -> "SourceProvenance":
             issue_tracker,
         )
 
+    def serialize(self) -> Dict[str, str]:
+        provenance = {}
+
+        if self.homepage is not None:
+            provenance["homepage"] = self.homepage
+
+        if self.issue_tracker is not None:
+            provenance["issue-tracker"] = self.issue_tracker
+
+        return provenance
+
 
 # _Scope():
 #