keep .hg* for .hgignore and .hgtags

With a single underscore, the "link target" system is used, resulting in
conflicts when the same version name is shared by 2 repos.

docker_build: copy modules (defined in setup.cfg in superproject to a new directory for docker to do)

create_documentation: only create documentation for used modules, reusing the setup.cfg key defined for create_archive if it is present

excluding the demo data, which can trigger bugs.

tag.

correctly get branch name (.git/hg are in the module directory), and use default if no branch information available