Fix ports depends-list.sh

[andrew_tao173.riddles.org.uk]

Summary

depends-list.sh fails in two ways: one is a simple quoting bug (which would have been detected by shellcheck) which causes empty FLAVOR variables to be propagated; the other is that de-duplication is being done without regard for flavors.

The upshot of these is that where a port depends on a specific flavor (first bug) or on multiple flavors (second bug, usually caused by build dependencies) of another port, the constructed dependency list is incomplete, and so -recursive targets do not do all the required work. (This is especially annoying for fetch-recursive.)

See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=269982 on this exact issue which has been open for three months without attention; this diff addresses both the original bug and my followup comment.

Test Plan

Extensively tested locally (I rely on fetch-recursive to fetch all distfiles).

[rene]

Looks fine to me, unless I'm missing something. @pizzamig do you want to take this as you own the PR too?