`os.walk` is a generator that can iteratively navigate from the
specified path, top-bottom. However, most of the calls to `os.walk` in
the project cast the call to `list()`, which makes it traverse the path
and recursively find all nested directories.
This is commonly not needed, as we end up just using a `[0]` index to
only access the root path.
This change adds a few utils that simplifies listing files/directories,
and by default does it non-recursively. Performance gains shouldn't be
noticeable in systems with high-speed storage, but we can avoid the edge
cases of users having too many nested directories, by avoiding unneeded
I/O.
