shell support: make `which spack` output intelligible (#19256)

Zsh and newer versions of bash have a builtin `which` function that will
show you if a command is actually an alias or a function. For functions,
the entire function is printed, and our `spack()` function is quite long.
Instead of printing out all that, make the `spack()` function a wrapper
around `_spack_shell_wrapper()`, and include some no-ops in the
definition so that users can see where it was created and where Spack is
installed.

Here's what the new output looks like in zsh:

```console
$ which spack
spack () {
	: this is a shell function from: /Users/gamblin2/src/spack/share/spack/setup-env.sh
	: the real spack script is here: /Users/gamblin2/src/spack/bin/spack
	_spack "$@"
	return $?
}
```

Note that `:` is a no-op in Bourne shell; it just discards anything after
it on the line. We use it here to embed paths in the function definition
(as comments are stripped).

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