Make whichLanguage -l still respect availability

[blast-hardcheese]

Why

In a pnpm project, when asking for -l python, I get python3-uv even though uv is not configured for that project:

$ upm which-language -l python
python3-uv
$ upm which-language -a  # Note, does not include python3-uv
nodejs-pnpm

We were not checking whether the backend is available before printing

What changed

Make which-languages -l ... respect the semantics of which-languages -a

Test plan

Describe what you did to test this change to a level of detail that allows your reviewer to test it