Fix proxydeps QA check when multiple flavor of a package are installed.

Previously it would do this:

$ pkg which -q -o /usr/local/lib/libphonon4qt5.so
multimedia/phonon
$ pkg pkg annotate -q -S multimedia/phonon flavor
qt4
qt5

And that would break things in an interesting way.

This changes makes it use the package name of the required file, which
is unique.

Note that this problem would probably only ever happen on a live system.
I was not able to find a single port that would trigger this it in a
clean environment (poudriere testport).

PR: 231332
Submitted by: dbn