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/posix-source-accessor.hh

@@ -7,7 +7,7 @@ namespace nix {
 /**
  * A source accessor that uses the Unix filesystem.
  */
-struct PosixSourceAccessor : SourceAccessor
+struct PosixSourceAccessor : virtual SourceAccessor
 {
     /**
      * The most recent mtime seen by lstat(). This is a hack to

src/libutil/source-accessor.cc

@@ -7,6 +7,7 @@ static std::atomic<size_t> nextNumber{0};
 
 SourceAccessor::SourceAccessor()
     : number(++nextNumber)
+    , displayPrefix{"«unknown»"}
 {
 }
 
@@ -55,9 +56,15 @@ SourceAccessor::Stat SourceAccessor::lstat(const CanonPath & path)
         throw Error("path '%s' does not exist", showPath(path));
 }
 
+void SourceAccessor::setPathDisplay(std::string displayPrefix, std::string displaySuffix)
+{
+    this->displayPrefix = std::move(displayPrefix);
+    this->displaySuffix = std::move(displaySuffix);
+}
+
 std::string SourceAccessor::showPath(const CanonPath & path)
 {
-    return path.abs();
+    return displayPrefix + path.abs() + displaySuffix;
 }
 
 }

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);
 };