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.