Add support for aliases (#17229)

Add a new config section: `config:aliases`, which is a dictionary mapping aliases
to commands.

For instance:


```yaml
config:
    aliases:
        sp: spec -I
```

will define a new command `sp` that will execute `spec` with the `-I`
argument. 

Aliases cannot override existing commands, and this is ensured with a test.

We cannot currently alias subcommands. Spack will warn about any aliases
containing a space, but will not error, which leaves room for subcommand
aliases in the future.

---------

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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