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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