fix: fix is_in_apt function for virtual packages

[meteyou]

This PR fix the is_in_apt function for virtual packages. For example policykit-1 doesn't exist anymore in Debian Trixie and the output of apt-cache policy policykit-1 is:

policykit-1:
  Installed: (none)
  Candidate: (none)
  Version table:

This output creates a false positive with the old is_in_apt like here: https://github.com/meteyou/MainsailOS-dev/actions/runs/15515686418/job/43682280429#step:13:195

Here is a test run with the new is_in_apt: https://github.com/meteyou/MainsailOS-dev/actions/runs/15516654843/job/43684788202#step:13:206

An alternative version would be to parse the Candidate output if it is != (none)

[foosel]

Frankly, that solution of yours also strikes me as the cleaner way to get this info 👍