Restore builtins.pathExists behavior on broken symlinks

Commit 83c067c0fa changed `builtins.pathExists`
to resolve symlinks before checking for existence. Consequently, if the path
refers to a symlink itself, existence of the target of the symlink (instead of
the symlink itself) was checked. Restore the previous behavior by skipping
symlink resolution in the last component.

(cherry picked from commit 89e21ab4bd)

src/libutil/source-path.hh

@@ -11,6 +11,22 @@
 
 namespace nix {
 
+enum class SymlinkResolution {
+    /**
+     * Resolve symlinks in the ancestors only.
+     *
+     * Only the last component of the result is possibly a symlink.
+     */
+    Ancestors,
+
+    /**
+     * Resolve symlinks fully, realpath(3)-style.
+     *
+     * No component of the result will be a symlink.
+     */
+    Full,
+};
+
 /**
  * An abstraction for accessing source files during
  * evaluation. Currently, it's just a wrapper around `CanonPath` that
@@ -102,11 +118,11 @@ struct SourcePath
     bool operator<(const SourcePath & x) const;
 
     /**
-     * Resolve any symlinks in this `SourcePath` (including its
-     * parents). The result is a `SourcePath` in which no element is a
-     * symlink.
+     * Resolve any symlinks in this `SourcePath` according to the
+     * given resolution mode.
      */
-    SourcePath resolveSymlinks() const;
+    SourcePath resolveSymlinks(
+        SymlinkResolution mode = SymlinkResolution::Full) const;
 };
 
 std::ostream & operator << (std::ostream & str, const SourcePath & path);