This is mostly a bugfix update with improvements for building modules and flatpaks

• Set configuration in case of "clone --branches" as well
• Send source mtime to dist-git
• Specify package manager for mock-config
• Add contributing guide
• Validate the module build optional argument when parsing the argument
• Add config options to parse the base module (e.g. platform) stream from the dist-git branch and apply a buildrequire override
• Add the ability to pass in buildrequire and require overrides on a module build
• Raise an error if the module build command receives optional arguments that conflict
• Add flatpak-build subcommand