fix(cli): realPath honors non-unc win32 path separator

[kwonoj]

• closes realPathSync returns paths with UNIX "/" separator on Windows #5685

Description:

This PR tries to fix path separator returned by realPah.

It looks like issue stemmed from rust-lang/rust#42869. Canonicalize on windows returns UNC path, stripping unc host (https://github.com/denoland/deno/blob/master/cli/ops/fs.rs?rgh-link-date=2020-06-02T04%3A57%3A06Z#L584) will leave rest of path do not honor win32 path separator. Given std canonicalize does not provide workaround yet, stripping UNC via drop-in replacement (gitlab.com/kornelski/dunce) seems the easiest way to fix instead of re-implementing UNC stripping logics.

It may possible using another crate may not be desired or other way to workaround may exist.

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[piscisaureus]

'dunce' definitely seems to be engineered very diligently and the current canonicalize situation is indeed terrible, so I'm leaning towards landing this. That said, I do have some comments, maybe other windows users can chip in with their opinion on things:

• I don't know how I feel about dunce encoding all these dos peculiarities such as that files can't be named aux.txt etc (and thus it will still canonicalize to \\?\C:\x\y\z\aux.txt). Although it is true in practice that most programs can't open a file with that name, in node we used the much "dumber" strategy of just stripping \\?\ and I don't believe very many people had issues with it.
• I remember from my node days that people that use subst didn't love it that realpath() changes the drive letter to the drive that contains the subst'ed directory. I think we might want to add an option (or even make it the default) to stay on the same drive if possible.
• For a complete solution it would also have to fix up network paths. With dunce I still get this:

  fn main() {
    let s = r#"\\localhost\c$\windows"#;
    println!("{:?}", dunce::canonicalize(s).unwrap());
    // Outputs: "\\\\?\\UNC\\localhost\\c$\\Windows"
  }

[nayeemrmn]

Sorry for the possibly dumb question. Isn't the cause of the wrong path separator simply the replacement in this line? What is it for?

[bartlomieju]

@nayeemrmn good catch, @piscisaureus PTAL

[piscisaureus]

I don't know what you guys think is wrong and what you expect.
I expect backslashes. So it was wrong and after this patch it is good.

[nayeemrmn]

@piscisaureus Can we fix this by removing the \\ -> / replacement (and maybe adapting the trim statement below), instead of bringing in a new dependency?