`path.EnumerateFiles`

[ForNeVeR]

We have the full family of path.GetFiles() with different overloads that return arrays, but we are currently missing path.EnumerateFiles that would return an IEnumerable. Let's have it.

While we are at it, let's correctly type it: it should return a collection of AbsolutePath, not strings.

Deprecate the GetFiles with the wrong return type.