Fixed runtimepath resolving

[Milly]

Vim-polyglot changes runtimepath, but...

• In Windows, resolve() does not do full normalization.
  Therefore, the path of vim-polyglot is not filtered, and registered more than once.
  ex.) runtimepath =>

  C:\Users\alice\.config\vim
  C:/Users/alice/.vimplugins/vim-polyglot
  C:\Users\alice\.vimplugins\vim-polyglot
  C:\tools\vim\vim82
  C:\Users\alice\.vimplugins\vim-polyglot/after
  C:/Users/alice/.vimplugins/vim-polyglot/after
  C:\Users\alice\.config\vim/after
  

  Fix) Substitute \ to / after resolved.
• Rewriting the runtimepath has too many side effects.
  ex.) Conflicts occur when the plugin-manager manages the paths.
  Fix) Resolved paths are used only for matching, and the original path representation is preserved.

[rene-descartes2021]

• Rewriting the runtimepath has too many side effects.
  ex.) Conflicts occur when the plugin-manager manages the paths.
  Fix) Resolved paths are used only for matching, and the original path representation is preserved.

I ran into this issue when using the Shougo/dein.vim package manager. dein.vim needs the index of its cache in the rtp to manage things. As soon as vim-polyglot is sourced (editing the rtp) dein.vim breaks as it no longer is able to find the index of its cache.

In my case I symlinked the cache to a tmpfs. vim-polyglot's resolve() rtp editing changes /home/browser/.vim/plugged/.cache/vimrc/.dein to /tmp/.vimcachebrowser/vimrc/.dein and dein.vim can't index /home/browser/.vim/plugged/.cache/vimrc/.dein within the rtp anymore and critical functionality breaks.

This PR fixes the issue on my Debian system.

I think this PR also fixes #769, who questions:

Is it necessary to resolve() each of the paths in the runtime path?