Remove UNC Prefix after Canonicalize on Windows

[charlespierce]

Closes #635
Closes #630
Closes #586

Info

• We use fs::canonicalize() in a few places to resolve relative paths and potential symlinks to the target file.
• On Windows, canonicalize returns a UNC path with \\?\ in the prefix, however those paths cause issues in a number of use-cases (see also std::fs::canonicalize returns UNC paths on Windows, and a lot of software doesn't support UNC paths rust-lang/rust#42869):
  • JS Scripts that append paths using concatenation instead of path.join can cause issues, since UNC paths don't support / as a separator (see windows global command issue #635 and gulp-cli does not appear to work #586)
  • We incorrectly fail to remove the Volta install directory from the PATH variable when doing a passthrough, because the comparison between C:\...\Volta and \\?\C:\...\Volta fails to match. This can cause an infinite loop of calls to the built-in shims, since they aren't removed from the PATH (see Infinite loop when running npm i #630).

Changes

• Updated all calls to canonicalize to use dunce
• Dunce inspects the PATH and strips off the UNC prefix if it is safe (following the Windows conventions for when the two paths are equivalent).
  • On non-Windows systems, dunce::canonicalize is an inline passthrough to fs::canonicalize, so Unix remains unaffected.
• Also cleaned up some redundant clones that Clippy found.

Tested

• Tested all three bugs caused by UNC paths (gulp-cli, ember-cli, and calling npm i with passthrough.
• Confirmed that each was reproducible prior to and fixed after the changes.

[chriskrycho]

Amazing detective work, and awesome fix! ⚡️

[wycats]

Great work!

If I recall correctly from when I was helping with this API, the UNC prefix allows the use of very long paths, but that's less of an issue on modern node and modern Windows.