The new `namespace apply` feature that allows for passing a namespace
specification file detects the difference between an empty namespace
and a namespace specification by checking if the file exists. For most
cases, the file will have an extension like `.hcl` and so there's
little danger that a user will apply a file spec when they intended to
apply a file name.
But because directory names typically don't include an extension,
you're much more likely to collide when trying to `namespace apply` by
name only, and then you get a confusing error message of the form:
Failed to read file: read $namespace: is a directory
Detect the case where the namespace name collides with a directory in
the current working directory, and skip trying to load the directory.
