Get rid of CanonPath::fromCwd

As discussed in the last Nix team meeting (2024-02-95), this method
doesn't belong because `CanonPath` is a virtual/ideal absolute path
format, not used in file systems beyond the native OS format for which a
"current working directory" is defined.

Progress towards #9205

src/libexpr/eval.hh

@@ -372,6 +372,11 @@ public:
      */
     SourcePath rootPath(CanonPath path);
 
+    /**
+     * Variant which accepts relative paths too.
+     */
+    SourcePath rootPath(PathView path);
+
     /**
      * Allow access to a path.
      */