Bug#946724: dh-elpa: fails on native package versions with '+'

[Roland Rosenfeld <roland@spinnaker.de>]

Package: dh-elpa
Version: 2.0.2
Severity: normal

Dear Maintainer,

trying to build lbdb with version 0.49+foo fails when calling
dh_elpa with the following message:

   dh_elpa
Invalid version syntax: `0.49+foo'
dh_elpa: emacs -batch -Q -l package --eval (add-to-list 'package-directory-list "/usr/share/emacs/site-lisp/elpa") --eval (add-to-list 'package-directory-list "/usr/share/emacs/site-lisp/elpa-src") -f package-initialize -l dh-elpa.el -f dhelpa-batch-install-directory debian/elpa-lbdb//usr/share/emacs/site-lisp/elpa-src /build/lbdb-0.49+foo/debian/.debhelper/elpa/lbdb /build/lbdb-0.49+foo/debian/.debhelper/elpa 1552316645 returned exit code 255
make: *** [debian/rules:15: binary] Error 255

(see https://salsa.debian.org/roland/lbdb/-/jobs/455490 for another
example).

I can reproduce the same behavior with dh-elpa 1.16 on buster as well
as with 2.0.2 on sid.

Using the "normal" version number 0.49 doesn't trigger the issue but
changing it to 0.49+foo or the like always triggers above failure.

According to deb-version(7) and policy chapter 5.6.12 the upstream
version of a package is allowed to contain a plus sign, so 0.49+foo is
a valid version number.

Greetings
Roland