Windows: Remove the \\?\ prefix from paths

[ChrisDenton]

When a path is canonicalized on Windows, a \\?\ prefix is used. However, it appears that cmake can have problems with that. This simply attempts to remove the prefix as we do still want to use absolute path here due to #200 cc @tgross35.

Fixes #258

[tgross35]

Cmake should probably support \\?\, right? Seems like this may be worth an issue on their end. Cc @vron if you may want to open one.

[ChrisDenton]

Cmake should probably support \\?\, right?

I would hope so, yes! Though it's possible it's some dependency that's ultimately to blame.

[kornelski]

This breaks long paths and file path components that have different meaning under the DOS path parser.

You can't remove the verbatim prefix without checking whether the rest of the path is actually compatible. \\?\C:\ may not be usable as C:\ paths.

See what path validation is done in the dunce crate, or just use that crate.

[ChrisDenton]

I am aware. The only cases that could practical fail here are:

• A directory component ends in a single dot
• The path ends in a dot or a space
• The basename is equivalent to a device name (drive paths only)

However, these cases have always broken because we use the build_dir path to set the current directory, which doesn't support such paths. The absolute best we could do here is error with good diagnostics. But that's only helpful if the path made it this far without breaking something.