This removes fish command completions (but keeps argument completions)
for commands in the `COMPLETIONS_EXCLUSION_LIST` list.
It also removes unnecessary exclusions of `*instal` aliases in command
suggestions in bash and fish completions, since those were removed from
`internal_commands_list.txt` in Homebrew#10229. This makes
`COMPLETIONS_EXCLUSION_LIST` the single source of truth for commands to
exclude from completions.
As of `brew` 4.0.2, sourcing the Bash completions with `set -eu` causes a
failure if `POSIXLY_CORRECT` is not declared. The check will now
properly handle this situation.
I'm not sure if the resulting code is POSIX-compliant, but it prevents a
breaking change until a better solution can be found.
