Dgit.pm: Allow uppercase (ascii) letters in multi-orig components

Dgit.pm: Allow uppercase (ascii) letters in multi-orig components

We didn't recognise files like
   firefox-esr_60.4.0esr.orig-l10n-sv-SE.tar.bz2
as orig files.  As a result, dgit would see all the things in the
corresponding subdirectory as having been added in Debian changes,
leading it making inaccurate complaints and bombing out.

See also #918438 against policy, where I (effectively) argue that this
is a design error in the `3.0 (quilt)' format.

Closes: #916926
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>