feat: Optimized fs walker and util.IsEmptyDir

[idsulik]

Description

1. Optimized file system walker, it used to make 80k(in my case) IO operations, because in the condition util.IsEmptyDir was before !info.IsDir(), though there is no reason to make IO operation to check is the directory empty if it's not a directory.
2. Optimized utils.IsEmptyDir, it used to fetch all information about the directory while we just need to check if it has anything in it, so now it fetches only name

User facing changes
before - It makes N extra IO operations, where N - is docker context files count
after - It doesn't make extra IO operations.

p.s. this changes optimized fs walker by 50% on my laptop

[ericzzzzzzz]

@idsulik Thanks for the optimization!