fs,module: add module-loader-only realpath cache

Reintroduce a realpath cache with the same mechanisms which existed
before b488b19e
(`fs: optimize realpath using uv_fs_realpath()`), but only for
the synchronous version and with the cache being passed as a
hidden option to make sure it is only used internally.

The cache is hidden from userland applications because it has been
decided that fully reintroducing as part of the public API might stand
in the way of future optimizations.

PR-URL: https://github.com/nodejs/node/pull/8100


Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: James M Snell <jasnell@gmail.com>