cli: detect directory when applying namespace spec file

[tgross]

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.

---

No changelog entry, as this feature shipped for the first time in Nomad 1.3.0-beta.1 (ref #11813)

Before:

$ nomad namespace apply prod
Successfully applied namespace "prod"!

$ nomad namespace apply ./namespace.hcl
Successfully applied namespace "staging"!

$ nomad namespace apply dev
Failed to read file: read dev: is a directory

After:

$ nomad namespace apply prod
Successfully applied namespace "prod"!

$ nomad namespace apply ./namespace.hcl
Successfully applied namespace "staging"!

$ nomad namespace apply dev
Successfully applied namespace "dev"!

[DerekStrickland]

LGTM

[shoenig]

LGTM!

[github-actions]

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