This completes to `spack concretize`:
```
spack conc<tab>
```
but this still gets hung up on the difference between `concretize` and `concretise`:
```
spack -e . conc<tab>
```
We were checking `"$COMP_CWORD" = 1`, which tracks the word on the command line
including any flags and their args, but we should track `"$COMP_CWORD_NO_FLAGS" = 1` to
figure out if the arg we're completing is the first real command.
Smart alias completion introduced in #39499 wasn't as smart as it needed to be, and
would complete any invalid command prefix and some env names with alias names.
- [x] don't complete aliases if there are no potential completions
e.g., don't convert `spack isnotacommand` -> `spack concretize`
- [x] don't complete with an aliases if we're not looking at a top-level subcommand.
`compgen -W` does not behave the same way in zsh as it does in bash; it seems not to
actually generate the completions we want.
- [x] add a zsh equivalent and `_compgen_w` to abstract it away
- [x] use `_compgen_w` instead of `compgen -W`
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
```
