Use "policy" command as a more reliable way to detect missing packages

[guss77]

The output from apt-cache policy is much more strict and limited in its variability and content and meant for machine parsing, so it should be preferred over apt-cache show when trying to detect the availability of a package.

This change fixes issue #995 . Though that issue is already fixed in main, I believe the current fix - that greps the output of apt-cache show is not reliable enough and could get us in trouble in the future.

[almereyda]

This will be good to consider in the next release, which has already grown suitably, with activity on main declining in the last month, inviting for cutting a new version soon:

• 1.5.0.2...main
• 1.5.0.2...bf65e62 as of today

[89luca89]

Thanks! :)