irregular mode file checks for Windows symlinks

[notchairmk]

Adds checks for os.ModeIrregular anywhere where we were checking symlinks. This is required as of Go 1.23, additional discussion in golang/go#63703

Fixes hashicorp/terraform#36163

[dduzgun-security]

Would it be better if we handle ModeIrregular in it's own condition block instead of the symlink one since ModeIrregular is for non-regular file; nothing else is known about this file which isn't technically a symlink and it could make things confusing?

[notchairmk]

hey @dduzgun-security what this basically boils down to is that Windows and now Go only treat files as symlinks, not directories. the go issue mentioned in the description has more details on this.

in the context of go-slug, there is no distinction between symlinked directories and files. we could make this more specific like m&os.ModeSymlink != 0 || (m&os.ModeIrregular != 0 && runtime.GOOS == 'windows') or by adding comments describing what's going on. but it doesn't feel like making this check separate from the os.ModeSymlink checks is preferable.

[notchairmk]

also just want to note that tests for Windows will fail on main already, they're not necessarily due to this change.

[dduzgun-security]

@notchairmk Thanks for the context, I would suggest a condition for Windows first by using the check runtime.GOOS == 'windows' then following up with the symlink & irregular file check to reduce the impact of the change. This library is quite sensitive and used by many, targeting the change for the affected environment would reduce the risk of incidents.