Allow installation with PEP517

The installation of arbitrary (e.g. man pages) and required (e.g. /usr/share/namcap/namcap-tags) files is not covered by the PEP517 backend, as setuptools' data_files does not install outside of site-packages in that case.

For this reason I propose to include all required files such as parsepkgbuild.sh and namcap-tags in the namcap python package (following pypa upstream recommendations). We likely need to provide e.g. a Makefile for building/installing using PEP517 and installing arbitrary data such as man page (with a dedicated target).