The voodoo that checks this now keeps track of which packages caused
what, and in addition to the list FORCED_PKGSRC of packages that have
been forced to non-builtin, it also produces a list FORCED_PKGSRC_REASONS
of the form pkg:causing-pkg{,causing-pkg...}.
(which means that USE_BUILTIN.pkg has been set to no because
USE_BUILTIN.causing-pkg is set to no.)
This could probably just be one list but I'm not sure if anything is
relying on the format of the current FORCED_PKGSRC.
702361dd1b