Added a check that ensures that all types in makevars.map actually exist
in the code, avoiding "Type not found" fatal errors at runtime. In this
case, only the variables *_SPECIFIC_PKGS were affected.

The real problem that induced the above change was the unknown type
"List of Pathname", which was a bug in the pkglint code itself,
introduced three years ago (revision 1.574).

(rillig)