completion: add alias handling

Bash completion is now smarter about handling aliases. In particular, if all completions
for some input command are aliased to the same thing, we'll just complete with that thing.

If you've already *typed* the full alias for a command, we'll complete the alias.

So, for example, here there's more than one real command involved, so all aliases are
shown:

```console
$ spack con
concretise    concretize    config        containerise  containerize
```

Here, there are two possibilities: `concretise` and `concretize`, but both map to
`concretize` so we just complete that:

```console
$ spack conc
concretize
```

And here, the user has already typed `concretis`, so we just go with it as there is only
one option:

```console
 spack concretis
concretise
```

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