Don't re-run the solver unnecessarily (#12119)

[tdammers]

Previously, the solver would be re-run whenever anything about the package description changed. This changes that so that only the relevant bits (roughly: the information found in PInfo) is taken into account, ignoring other changes to the package description.

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)