Support yaml paths anywhere specs are handled on CLI (#12561)

Update command-line (CLI) parsing to understand references to yaml
files that store Spack specs. Where a file reference is encountered,
the full Spec in the file will be read in. A file reference may
appear anywhere that a spec could appear before. For example, if you
write "spack spec -y openmpi > openmpi.yaml" you may then install the
spec using the yaml file by running "spack install ./openmpi.yaml";
you can also refer to dependencies in this way (e.g.
"spack install foo^./openmpi.yaml").

There are two requirements for file references:

* A file path entered on the CLI must include a "/" even if the file
  exists in your current working directory. For example, if you
  create an openmpi.yaml file as above and run
  "spack install openmpi.yaml" from the same directory, it will
  report an error.
* A file path entered on the CLI must end with ".yaml"

This commit adds error messages to clearly inform the user of both
violations.

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