fix: avoid internal error for unicode path elements

[jedevc]

The issue (found at kubecon 🎉):

$ pwd
/tmp/work/fooé/test
$ dagger init --sdk=go --source=.
✘ ModuleSource.resolveContextPathFromCaller: String! 0.0s

Error: failed to get configured module: failed to get local root path: input: moduleSource.resolveContextPathFromCaller resolve: failed to find up root: failed to lstat .git: failed to create diff copy client: rpc error: code = Internal desc = rpc error: code = Internal desc = header key "dir-name" contains value with non-printable ASCII characters

A similar issue was resolved in buildkit a while back: moby/buildkit#3946

However, because we have our own filesync implementation, we need our own fix. To do this, I've just imported the helpers from upstream, in a copy-paste job (since they're not exported, and they probably shouldn't be, it's not an external implementation detail) - this impl is tested pretty thoroughly upstream and in the last buildkit release.

[jedevc]

Chucking this into the milestone, since it's a reasonably trivial fix.

[gerhard]

💪 🚀