Parser: Respect the accessor of the source file for relative paths

Previously we only returned paths in rootFS, which is wrong and only
worked because currently all our source trees are in rootFS.

src/libexpr/parser.y

@@ -359,11 +359,18 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath(std::string_view{$1.p, $1.l}, state->basePath.path.abs()));
+    std::string_view literal({$1.p, $1.l});
+    Path path(absPath(literal, state->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
-    if ($1.p[$1.l-1] == '/' && $1.l > 1)
-      path += "/";
-    $$ = new ExprPath(ref<SourceAccessor>(state->rootFS), std::move(path));
+    if (literal.size() > 1 && literal.back() == '/')
+      path += '/';
+    $$ =
+        /* Absolute paths are always interpreted relative to the
+           root filesystem accessor, rather than the accessor of the
+           current Nix expression. */
+        literal.front() == '/'
+        ? new ExprPath(state->rootFS, std::move(path))
+        : new ExprPath(state->basePath.accessor, std::move(path));
   }
   | HPATH {
     if (state->settings.pureEval) {