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>

etc/spack/defaults/config.yaml

@@ -229,3 +229,11 @@ config:
   flags:
     # Whether to keep -Werror flags active in package builds.
     keep_werror: 'none'
+
+  # A mapping of aliases that can be used to define new commands. For instance,
+  # `sp: spec -I` will define a new command `sp` that will execute `spec` with
+  # the `-I` argument. Aliases cannot override existing commands.
+  aliases:
+    concretise: concretize
+    containerise: containerize
+    rm: remove