Add the option to display package URLs in the search results

[jdholtz]

Fixes #2303. The URLs also show for packages in other databases too (not just AUR packages).

Providing the --showpackageurls flag in your config file or during a search (e.g. yay --showpackageurls linux) displays the URL of each package under its description, which the user can then click to view in their web browser.

Let me know if you want me to make constants for the website prefixes, https://archlinux.org/packages and https://aur.archlinux.org/packages, to make them easier to change in the future.

I can also change 'packageurls' to another name if you see one that fits better.

[stale]

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