debian: Vendor PyMuPDf when building Debian package

Install PyMuPDF under ./dangerzone/vendor, right before we build the
.deb package. We vendor PyMuPDF just for Debian, since the provided
versions don't have OCR support enabled.

Currently, we don't use PyMuPDf on the host, but this will change once
we fully implement the on-host conversion feature.

Refs #625

debian/rules

@@ -1,7 +1,13 @@
 #!/usr/bin/make -f
 export PYBUILD_NAME=dangerzone
 export DEB_BUILD_OPTIONS=nocheck
+export PYBUILD_INSTALL_ARGS=--install-lib=/usr/lib/python3/dist-packages
+export PYTHONDONTWRITEBYTECODE=1
+export DH_VERBOSE=1
 
 %:
 	dh $@ --with python3 --buildsystem=pybuild
 
+override_dh_builddeb:
+	./install/linux/vendor-pymupdf.py --dest debian/dangerzone/usr/lib/python3/dist-packages/dangerzone/vendor/
+	dh_builddeb $@