Draft: fix: rfc52: replace ad-hoc tests with pkgctl license check

This is a follow-up MR to !854 (merged).

Security notice

DO NOT MERGE unless pkgctl license check has been refactored to parse .SRCINFO instead of sourcing the user-provided PKGBUILD.

For details, see: !854 (comment 291185) by @anthraxx.

• Wait until !854 (merged) is merged
• Wait until pkgctl license check (devtools!322 (merged)) has been refactored
• Wait until new devtools upstream release has been cut
• Wait until devtools has landed in extra
• Rebase on master

/cc @artafinde, @gromit, @svenstaro