Move the internally defined symbol `fs.realpathCacheKey` to
the internal fs module, where it’s more appropriate.
The symbol was recently added in c084287a60, 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>
The modules in lib/internal are intended for internal use in Node.js core only, and are not accessible with require() from user modules.
These are subject to change at any time. Reliance on these modules outside of core is not supported in any way.