Improve SourceAccessor path display

Backported from lazy-trees. This allows SourceAccessors to show the
origin of the accessor. E.g. we now get

  copying '«git+https://github.com/blender/blender.git?ref=refs/heads/main&rev=4edc1389337dd3679ff66969c332d2aff52e1992»/' to the store

instead of

  copying '/' to the store

src/libutil/source-accessor.hh

@@ -17,6 +17,8 @@ struct SourceAccessor
 {
     const size_t number;
 
+    std::string displayPrefix, displaySuffix;
+
     SourceAccessor();
 
     virtual ~SourceAccessor()
@@ -117,6 +119,8 @@ struct SourceAccessor
         return number < x.number;
     }
 
+    void setPathDisplay(std::string displayPrefix, std::string displaySuffix = "");
+
     virtual std::string showPath(const CanonPath & path);
 };