L::C::Package: Work around a bug

After commit 6460c51d, I noticed that some tests (e.g.
deb-format-wrong-order) started to produce use of uninitialised
values.
  It boiled down to $file->dirname being undef for the "usr/" entry
and strangely enough, only that entry seemed to trigger that problem.

Work around this problem by unconditionally and immediately set the
dirname of all entries again.  We will replace it later to share the
varable and thereby save memory.

Signed-off-by: Niels Thykier <niels@thykier.net>