Add passing options via individual binary package headers

[wodny]

Allow to specify dh-virtualenv options per binary package like so (debian/control):

Package: foo
Conflicts: foo-http
[...]
Package: foo-http
Conflicts: foo
X-Dh-Virtualenv-Extras: http
X-Dh-Virtualenv-Install-Suffix: foo

With the following in setup.cfg:

[options.extras_require]
http = aiohttp ~= 3.8

I wanted to build two debian packages with different extras and did not find an easier way. If this method seems reasonable and gets accepted, I will write and push documentation and some tests.

(cherry picked from commit a43d163)

[nailor]

Hey,

sorry for a late reply here but definitely something that could be included in the package if you provide tests and documentation on it!