Added cache copying support for worktrees (#95646)
This lets next detect if it is in a worktree and find the main repo and
copy turbopack caches for build or dev to make initial build faster. I
wasn't sure about commiting tests for this and how exactly we'd won't to
go about this with the git requirement. I also could do some unit tests,
but following the library conventions there wasn't a nice abstracted out
file system. So I did various tests locally. If anyone has good ideas on
what kind of testing totally opened to it.
Also left a comment that I don't look in other worktrees but instead the
root project. Not sure if this is the right answer or not. But works for
the patterns I think people typically follow.
