Replace which with a manual implementation using SearchPathW

[zadjii-msft]

This pull request removes our dependency on which, which Kenny so helpfully pointed out we don't need: https://gist.github.com/kennykerr/a4375597c7507182570576cf9e7b6ae5

[DHowett]

Can you split up the SFI feed changes and the which crate changes so we aren’t blocking one for the other? 😄

[zadjii-msft]

(sshhh bot, I was on vacation)