Spack command extensions: error-handling (#13635)

Generally speaking, errors that are encountered when attempting to load
command extensions now terminate the running Spack instance.

* Added new exceptions `spack.cmd.PythonNameError` and
  `spack.cmd.CommandNameError`.
* New functions `spack.cmd.require_python_name(pname)` and
  `spack.cmd.require_cmd_name(cname)` check that `pname` and `cname`
  respectively meet requirements, throwing the appropriate error if not.
* `spack.cmd.get_module()` uses `require_cmd_name()` and passes through
  exceptions from module load attempts.
* `spack.cmd.get_command()` uses `require_cmd_name()` and invokes
  `get_module()` with the correct command-name form rather than the
  previous (incorrect) Python name.
* Added New exceptions `spack.extensions.CommandNotFoundError` and
  `spack.extensions.ExtensionNamingError`.
* `_extension_regexp` has a new leading underscore to indicate expected
  privacy.
* `spack.extensions.extension_name()` raises an `ExtensionNamingError`
  rather than using `tty.warn()`.
* `spack.extensions.load_command_extension()` checks command source
  existence early and bails out if missing. Also, exceptions raised by
  `load_module_from_file()` are passed through.
* `spack.extensions.get_module()` raises `CommandNotFoundError` as
  appropriate.
* Spack `main()` allows `parser.add_command()` exceptions to cause
  program end.

Tests:

* More common boilerplate has been pulled out into fixtures including
  `sys.modules` dictionary cleanup and resource-managed creation of a
  simple command extension with specified contents in the source file
  for a single named command.
* "Hello, World!" test now uses a command named `hello-world` instead of
  `hello` in order to verify correct handling of commands with hyphens.
* New tests for:
  * Missing (or misnamed) command.
  * Badly-named extension.
  * Verification that errors encountered during import of a command are
    propagated upward.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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