repos: Add repo source_for_dependency method

... to conviniently learn what sources you need to build
to get something that provides this dependency, by name.

repos_cmp/repos.py

@@ -201,6 +201,10 @@ class Repository:
             if item[0].is_provide_for(dependency):
                 yield item
 
+    def source_for_dependency(self, dependency):
+        return [dep[1].source_rpm
+                for dep in self.reverse_prov[dependency]]
+
     def _unmets(self, packages):
         result = []
         for pkg in packages: