fs: cache non-symlinks in realpathSync.

Extend `fs.realpathSync` to cache the results for paths that are not
symlinks in addition to caching symlink mappings.

PR-URL: https://github.com/nodejs/node/pull/10253


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>