fs: export `realpathCacheKey` from `internal/fs`

Move the internally defined symbol `fs.realpathCacheKey` to
the internal fs module, where it’s more appropriate.

The symbol was recently added in c084287a, but since
`internal/fs` is only available in the v7.x branch, this
needs to be a separate follow-up change.

PR-URL: https://github.com/nodejs/node/pull/8862


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>